[Intel-gfx] [PATCH 0/2] x86/pat: fix querying available caching modes

[Intel-gfx] [PATCH 0/2] x86/pat: fix querying available caching modes

[Juergen Gross]

Fix some issues with querying caching modes being available for memory
mappings.

This is a replacement for the patch of Jan sent recently:

https://lists.xen.org/archives/html/xen-devel/2022-04/msg02392.html

Juergen Gross (2):
  x86/pat: fix x86_has_pat_wp()
  x86/pat: add functions to query specific cache mode availability

 arch/x86/include/asm/memtype.h           |  2 ++
 arch/x86/include/asm/pci.h               |  2 +-
 arch/x86/mm/init.c                       | 24 ++++++++++++++++++++++--
 drivers/gpu/drm/i915/gem/i915_gem_mman.c |  8 ++++----
 4 files changed, 29 insertions(+), 7 deletions(-)

-- 
2.35.3

[Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Juergen Gross]

Some drivers are using pat_enabled() in order to test availability of
special caching modes (WC and UC-). This will lead to false negatives
in case the system was booted e.g. with the "nopat" variant and the
BIOS did setup the PAT MSR supporting the queried mode, or if the
system is running as a Xen PV guest.

Add test functions for those caching modes instead and use them at the
appropriate places.

For symmetry reasons export the already existing x86_has_pat_wp() for
modules, too.

Fixes: bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
Fixes: ae749c7ab475 ("PCI: Add arch_can_pci_mmap_wc() macro")
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/include/asm/memtype.h           |  2 ++
 arch/x86/include/asm/pci.h               |  2 +-
 arch/x86/mm/init.c                       | 25 +++++++++++++++++++++---
 drivers/gpu/drm/i915/gem/i915_gem_mman.c |  8 ++++----
 4 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/memtype.h b/arch/x86/include/asm/memtype.h
index 9ca760e430b9..d00e0be854d4 100644
--- a/arch/x86/include/asm/memtype.h
+++ b/arch/x86/include/asm/memtype.h
@@ -25,6 +25,8 @@ extern void memtype_free_io(resource_size_t start, resource_size_t end);
 extern bool pat_pfn_immune_to_uc_mtrr(unsigned long pfn);
 
 bool x86_has_pat_wp(void);
+bool x86_has_pat_wc(void);
+bool x86_has_pat_uc_minus(void);
 enum page_cache_mode pgprot2cachemode(pgprot_t pgprot);
 
 #endif /* _ASM_X86_MEMTYPE_H */
diff --git a/arch/x86/include/asm/pci.h b/arch/x86/include/asm/pci.h
index f3fd5928bcbb..a5742268dec1 100644
--- a/arch/x86/include/asm/pci.h
+++ b/arch/x86/include/asm/pci.h
@@ -94,7 +94,7 @@ int pcibios_set_irq_routing(struct pci_dev *dev, int pin, int irq);
 
 
 #define HAVE_PCI_MMAP
-#define arch_can_pci_mmap_wc()	pat_enabled()
+#define arch_can_pci_mmap_wc()	x86_has_pat_wc()
 #define ARCH_GENERIC_PCI_MMAP_RESOURCE
 
 #ifdef CONFIG_PCI
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 71e182ebced3..b6431f714dc2 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -77,12 +77,31 @@ static uint8_t __pte2cachemode_tbl[8] = {
 	[__pte2cm_idx(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)] = _PAGE_CACHE_MODE_UC,
 };
 
-/* Check that the write-protect PAT entry is set for write-protect */
+static bool x86_has_pat_mode(unsigned int mode)
+{
+	return __pte2cachemode_tbl[__cachemode2pte_tbl[mode]] == mode;
+}
+
+/* Check that PAT supports write-protect */
 bool x86_has_pat_wp(void)
 {
-	return __pte2cachemode_tbl[__cachemode2pte_tbl[_PAGE_CACHE_MODE_WP]] ==
-	       _PAGE_CACHE_MODE_WP;
+	return x86_has_pat_mode(_PAGE_CACHE_MODE_WP);
+}
+EXPORT_SYMBOL_GPL(x86_has_pat_wp);
+
+/* Check that PAT supports WC */
+bool x86_has_pat_wc(void)
+{
+	return x86_has_pat_mode(_PAGE_CACHE_MODE_WC);
+}
+EXPORT_SYMBOL_GPL(x86_has_pat_wc);
+
+/* Check that PAT supports UC- */
+bool x86_has_pat_uc_minus(void)
+{
+	return x86_has_pat_mode(_PAGE_CACHE_MODE_UC_MINUS);
 }
+EXPORT_SYMBOL_GPL(x86_has_pat_uc_minus);
 
 enum page_cache_mode pgprot2cachemode(pgprot_t pgprot)
 {
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index 0c5c43852e24..f43ecf3f63eb 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -76,7 +76,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 	if (args->flags & ~(I915_MMAP_WC))
 		return -EINVAL;
 
-	if (args->flags & I915_MMAP_WC && !pat_enabled())
+	if (args->flags & I915_MMAP_WC && !x86_has_pat_wc())
 		return -ENODEV;
 
 	obj = i915_gem_object_lookup(file, args->handle);
@@ -757,7 +757,7 @@ i915_gem_dumb_mmap_offset(struct drm_file *file,
 
 	if (HAS_LMEM(to_i915(dev)))
 		mmap_type = I915_MMAP_TYPE_FIXED;
-	else if (pat_enabled())
+	else if (x86_has_pat_wc())
 		mmap_type = I915_MMAP_TYPE_WC;
 	else if (!i915_ggtt_has_aperture(to_gt(i915)->ggtt))
 		return -ENODEV;
@@ -813,7 +813,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
 		break;
 
 	case I915_MMAP_OFFSET_WC:
-		if (!pat_enabled())
+		if (!x86_has_pat_wc())
 			return -ENODEV;
 		type = I915_MMAP_TYPE_WC;
 		break;
@@ -823,7 +823,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
 		break;
 
 	case I915_MMAP_OFFSET_UC:
-		if (!pat_enabled())
+		if (!x86_has_pat_uc_minus())
 			return -ENODEV;
 		type = I915_MMAP_TYPE_UC;
 		break;
-- 
2.35.3

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Jan Beulich]

On 03.05.2022 15:22, Juergen Gross wrote:
> Some drivers are using pat_enabled() in order to test availability of
> special caching modes (WC and UC-). This will lead to false negatives
> in case the system was booted e.g. with the "nopat" variant and the
> BIOS did setup the PAT MSR supporting the queried mode, or if the
> system is running as a Xen PV guest.

While, as per my earlier patch, I agree with the Xen PV case, I'm not
convinced "nopat" is supposed to honor firmware-provided settings. In
fact in my patch I did arrange for "nopat" to also take effect under
Xen PV.

> Add test functions for those caching modes instead and use them at the
> appropriate places.
> 
> For symmetry reasons export the already existing x86_has_pat_wp() for
> modules, too.
> 
> Fixes: bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
> Fixes: ae749c7ab475 ("PCI: Add arch_can_pci_mmap_wc() macro")
> Signed-off-by: Juergen Gross <jgross@suse.com>

I think this wants a Reported-by as well.

> --- a/arch/x86/include/asm/pci.h
> +++ b/arch/x86/include/asm/pci.h
> @@ -94,7 +94,7 @@ int pcibios_set_irq_routing(struct pci_dev *dev, int pin, int irq);
>  
>  
>  #define HAVE_PCI_MMAP
> -#define arch_can_pci_mmap_wc()	pat_enabled()
> +#define arch_can_pci_mmap_wc()	x86_has_pat_wc()

Besides this and ...

> --- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> @@ -76,7 +76,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
>  	if (args->flags & ~(I915_MMAP_WC))
>  		return -EINVAL;
>  
> -	if (args->flags & I915_MMAP_WC && !pat_enabled())
> +	if (args->flags & I915_MMAP_WC && !x86_has_pat_wc())
>  		return -ENODEV;
>  
>  	obj = i915_gem_object_lookup(file, args->handle);
> @@ -757,7 +757,7 @@ i915_gem_dumb_mmap_offset(struct drm_file *file,
>  
>  	if (HAS_LMEM(to_i915(dev)))
>  		mmap_type = I915_MMAP_TYPE_FIXED;
> -	else if (pat_enabled())
> +	else if (x86_has_pat_wc())
>  		mmap_type = I915_MMAP_TYPE_WC;
>  	else if (!i915_ggtt_has_aperture(to_gt(i915)->ggtt))
>  		return -ENODEV;
> @@ -813,7 +813,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
>  		break;
>  
>  	case I915_MMAP_OFFSET_WC:
> -		if (!pat_enabled())
> +		if (!x86_has_pat_wc())
>  			return -ENODEV;
>  		type = I915_MMAP_TYPE_WC;
>  		break;
> @@ -823,7 +823,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
>  		break;
>  
>  	case I915_MMAP_OFFSET_UC:
> -		if (!pat_enabled())
> +		if (!x86_has_pat_uc_minus())
>  			return -ENODEV;
>  		type = I915_MMAP_TYPE_UC;
>  		break;

... these uses there are several more. You say nothing on why those want
leaving unaltered. When preparing my earlier patch I did inspect them
and came to the conclusion that these all would also better observe the
adjusted behavior (or else I couldn't have left pat_enabled() as the only
predicate). In fact, as said in the description of my earlier patch, in
my debugging I did find the use in i915_gem_object_pin_map() to be the
problematic one, which you leave alone.

Jan

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Juergen Gross]

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On 04.05.22 10:31, Jan Beulich wrote:
> On 03.05.2022 15:22, Juergen Gross wrote:
>> Some drivers are using pat_enabled() in order to test availability of
>> special caching modes (WC and UC-). This will lead to false negatives
>> in case the system was booted e.g. with the "nopat" variant and the
>> BIOS did setup the PAT MSR supporting the queried mode, or if the
>> system is running as a Xen PV guest.
> 
> While, as per my earlier patch, I agree with the Xen PV case, I'm not
> convinced "nopat" is supposed to honor firmware-provided settings. In
> fact in my patch I did arrange for "nopat" to also take effect under
> Xen PV.

Depends on what the wanted semantics for "nopat" are.

Right now "nopat" will result in the PAT MSR left unchanged and the
cache mode translation tables be initialized accordingly.

So does "nopat" mean that the PAT MSR shouldn't be changed, or that
PAGE_BIT_PAT will never be set?

>> Add test functions for those caching modes instead and use them at the
>> appropriate places.
>>
>> For symmetry reasons export the already existing x86_has_pat_wp() for
>> modules, too.
>>
>> Fixes: bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
>> Fixes: ae749c7ab475 ("PCI: Add arch_can_pci_mmap_wc() macro")
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> I think this wants a Reported-by as well.

Okay.

> 
>> --- a/arch/x86/include/asm/pci.h
>> +++ b/arch/x86/include/asm/pci.h
>> @@ -94,7 +94,7 @@ int pcibios_set_irq_routing(struct pci_dev *dev, int pin, int irq);
>>   
>>   
>>   #define HAVE_PCI_MMAP
>> -#define arch_can_pci_mmap_wc()	pat_enabled()
>> +#define arch_can_pci_mmap_wc()	x86_has_pat_wc()
> 
> Besides this and ...
> 
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
>> @@ -76,7 +76,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
>>   	if (args->flags & ~(I915_MMAP_WC))
>>   		return -EINVAL;
>>   
>> -	if (args->flags & I915_MMAP_WC && !pat_enabled())
>> +	if (args->flags & I915_MMAP_WC && !x86_has_pat_wc())
>>   		return -ENODEV;
>>   
>>   	obj = i915_gem_object_lookup(file, args->handle);
>> @@ -757,7 +757,7 @@ i915_gem_dumb_mmap_offset(struct drm_file *file,
>>   
>>   	if (HAS_LMEM(to_i915(dev)))
>>   		mmap_type = I915_MMAP_TYPE_FIXED;
>> -	else if (pat_enabled())
>> +	else if (x86_has_pat_wc())
>>   		mmap_type = I915_MMAP_TYPE_WC;
>>   	else if (!i915_ggtt_has_aperture(to_gt(i915)->ggtt))
>>   		return -ENODEV;
>> @@ -813,7 +813,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
>>   		break;
>>   
>>   	case I915_MMAP_OFFSET_WC:
>> -		if (!pat_enabled())
>> +		if (!x86_has_pat_wc())
>>   			return -ENODEV;
>>   		type = I915_MMAP_TYPE_WC;
>>   		break;
>> @@ -823,7 +823,7 @@ i915_gem_mmap_offset_ioctl(struct drm_device *dev, void *data,
>>   		break;
>>   
>>   	case I915_MMAP_OFFSET_UC:
>> -		if (!pat_enabled())
>> +		if (!x86_has_pat_uc_minus())
>>   			return -ENODEV;
>>   		type = I915_MMAP_TYPE_UC;
>>   		break;
> 
> ... these uses there are several more. You say nothing on why those want
> leaving unaltered. When preparing my earlier patch I did inspect them
> and came to the conclusion that these all would also better observe the
> adjusted behavior (or else I couldn't have left pat_enabled() as the only
> predicate). In fact, as said in the description of my earlier patch, in
> my debugging I did find the use in i915_gem_object_pin_map() to be the
> problematic one, which you leave alone.

Oh, I missed that one, sorry.

I wanted to be rather defensive in my changes, but I agree at least the
case in arch_phys_wc_add() might want to be changed, too.

kvm_is_mmio_pfn() should not really matter at least for the Xen case.

With the other use cases in memtype.c I'm rather on the edge.

In case the x86 maintainers think those should be changed, too, I agree
that your approach might be the better one.


Juergen

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Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Jan Beulich]

On 04.05.2022 11:14, Juergen Gross wrote:
> On 04.05.22 10:31, Jan Beulich wrote:
>> On 03.05.2022 15:22, Juergen Gross wrote:
>>> Some drivers are using pat_enabled() in order to test availability of
>>> special caching modes (WC and UC-). This will lead to false negatives
>>> in case the system was booted e.g. with the "nopat" variant and the
>>> BIOS did setup the PAT MSR supporting the queried mode, or if the
>>> system is running as a Xen PV guest.
>>
>> While, as per my earlier patch, I agree with the Xen PV case, I'm not
>> convinced "nopat" is supposed to honor firmware-provided settings. In
>> fact in my patch I did arrange for "nopat" to also take effect under
>> Xen PV.
> 
> Depends on what the wanted semantics for "nopat" are.
> 
> Right now "nopat" will result in the PAT MSR left unchanged and the
> cache mode translation tables be initialized accordingly.
> 
> So does "nopat" mean that the PAT MSR shouldn't be changed, or that
> PAGE_BIT_PAT will never be set?

According to the documentation for the option ("Disable PAT (page
attribute table extension of pagetables) support") I'd say the latter.

Jan

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Thorsten Leemhuis]

On 20.05.22 16:48, Chuck Zmudzinski wrote:
> On 5/20/2022 10:06 AM, Jan Beulich wrote:
>> On 20.05.2022 15:33, Chuck Zmudzinski wrote:
>>> On 5/20/2022 5:41 AM, Jan Beulich wrote:
>>>> On 20.05.2022 10:30, Chuck Zmudzinski wrote:
>>>>> On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
>>>>>> On 5/20/2022 2:05 AM, Jan Beulich wrote:
>>>>>>> On 20.05.2022 06:43, Chuck Zmudzinski wrote:
>>>>>>>> On 5/4/22 5:14 AM, Juergen Gross wrote:
>>>>>>>>> On 04.05.22 10:31, Jan Beulich wrote:
>>>>>>>>>> On 03.05.2022 15:22, Juergen Gross wrote:
>>>>>>>>>>
>>>>>>>>>> ... these uses there are several more. You say nothing on why
>>>>>>>>>> those want
>>>>>>>>>> leaving unaltered. When preparing my earlier patch I did
>>>>>>>>>> inspect them
>>>>>>>>>> and came to the conclusion that these all would also better
>>>>>>>>>> observe the
>>>>>>>>>> adjusted behavior (or else I couldn't have left pat_enabled()
>>>>>>>>>> as the
>>>>>>>>>> only predicate). In fact, as said in the description of my
>>>>>>>>>> earlier
>>>>>>>>>> patch, in
>>>>>>>>>> my debugging I did find the use in i915_gem_object_pin_map()
>>>>>>>>>> to be
>>>>>>>>>> the
>>>>>>>>>> problematic one, which you leave alone.
>>>>>>>>> Oh, I missed that one, sorry.
>>>>>>>> That is why your patch would not fix my Haswell unless
>>>>>>>> it also touches i915_gem_object_pin_map() in
>>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>>>>>>>>
>>>>>>>>> I wanted to be rather defensive in my changes, but I agree at
>>>>>>>>> least
>>>>>>>>> the
>>>>>>>>> case in arch_phys_wc_add() might want to be changed, too.
>>>>>>>> I think your approach needs to be more aggressive so it will fix
>>>>>>>> all the known false negatives introduced by bdd8b6c98239
>>>>>>>> such as the one in i915_gem_object_pin_map().
>>>>>>>>
>>>>>>>> I looked at Jan's approach and I think it would fix the issue
>>>>>>>> with my Haswell as long as I don't use the nopat option. I
>>>>>>>> really don't have a strong opinion on that question, but I
>>>>>>>> think the nopat option as a Linux kernel option, as opposed
>>>>>>>> to a hypervisor option, should only affect the kernel, and
>>>>>>>> if the hypervisor provides the pat feature, then the kernel
>>>>>>>> should not override that,
>>>>>>> Hmm, why would the kernel not be allowed to override that? Such
>>>>>>> an override would affect only the single domain where the
>>>>>>> kernel runs; other domains could take their own decisions.
>>>>>>>
>>>>>>> Also, for the sake of completeness: "nopat" used when running on
>>>>>>> bare metal has the same bad effect on system boot, so there
>>>>>>> pretty clearly is an error cleanup issue in the i915 driver. But
>>>>>>> that's orthogonal, and I expect the maintainers may not even care
>>>>>>> (but tell us "don't do that then").
>>>>> Actually I just did a test with the last official Debian kernel
>>>>> build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
>>>>> applied. In fact, the nopat option does *not* break the i915 driver
>>>>> in 5.16. That is, with the nopat option, the i915 driver loads
>>>>> normally on both the bare metal and on the Xen hypervisor.
>>>>> That means your presumption (and the presumption of
>>>>> the author of bdd8b6c98239) that the "nopat" option was
>>>>> being observed by the i915 driver is incorrect. Setting "nopat"
>>>>> had no effect on my system with Linux 5.16. So after doing these
>>>>> tests, I am against the aggressive approach of breaking the i915
>>>>> driver with the "nopat" option because prior to bdd8b6c98239,
>>>>> nopat did not break the i915 driver. Why break it now?
>>>> Because that's, in my understanding, is the purpose of "nopat"
>>>> (not breaking the driver of course - that's a driver bug -, but
>>>> having an effect on the driver).
>>> I wouldn't call it a driver bug, but an incorrect configuration of the
>>> kernel by the user.  I presume X86_FEATURE_PAT is required by the
>>> i915 driver
>> The driver ought to work fine without PAT (and hence without being
>> able to make WC mappings). It would use UC instead and be slow, but
>> it ought to work.
>>
>>> and therefore the driver should refuse to disable
>>> it if the user requests to disable it and instead warn the user that
>>> the driver did not disable the feature, contrary to what the user
>>> requested with the nopat option.
>>>
>>> In any case, my test did not verify that when nopat is set in Linux
>>> 5.16,
>>> the thread takes the same code path as when nopat is not set,
>>> so I am not totally sure that the reason nopat does not break the
>>> i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
>>> returns true even when nopat is set. I could test it with a custom
>>> log message in 5.16 if that is necessary.
>>>
>>> Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
>>> to return true in 5.16 when the user requests nopat?
>> No, I'm not saying that. It was wrong for this construct to be used
>> in the driver, which was fixed for 5.17 (and which had caused the
>> regression I did observe, leading to the patch as a hopefully least
>> bad option).
>>
>>> I think that is
>>> just permitting a bad configuration to break the driver that a
>>> well-written operating system should not allow. The i915 driver
>>> was, in my opinion, correctly ignoring the nopat option in 5.16
>>> because that option is not compatible with the hardware the
>>> i915 driver is trying to initialize and setup at boot time. At least
>>> that is my understanding now, but I will need to test it on 5.16
>>> to be sure I understand it correctly.
>>>
>>> Also, AFAICT, your patch would break the driver when the nopat
>>> option is set and only fix the regression introduced by bdd8b6c98239
>>> when nopat is not set on my box, so your patch would
>>> introduce a regression relative to Linux 5.16 and earlier for the
>>> case when nopat is set on my box. I think your point would
>>> be that it is not a regression if it is an incorrect user configuration.
>> Again no - my view is that there's a separate, pre-existing issue
>> in the driver which was uncovered by the change. This may be a
>> perceived regression, but is imo different from a real one.

Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
comes to the "no regressions rule". Just took a quick look at quotes
from Linus
https://www.kernel.org/doc/html/latest/process/handling-regressions.html
and found this statement from Linus to back this up:

```
One _particularly_ last-minute revert is the top-most commit (ignoring
the version change itself) done just before the release, and while
it's very annoying, it's perhaps also instructive.

What's instructive about it is that I reverted a commit that wasn't
actually buggy. In fact, it was doing exactly what it set out to do,
and did it very well. In fact it did it _so_ well that the much
improved IO patterns it caused then ended up revealing a user-visible
regression due to a real bug in a completely unrelated area.
```

He said that here:
https://www.kernel.org/doc/html/latest/process/handling-regressions.html

The situation is of course different here, but similar enough.

> Since it is a regression, I think for now bdd8b6c98239 should
> be reverted and the fix backported to Linux 5.17 stable until
> the underlying memory subsystem can provide the i915 driver
> with an updated test for the PAT feature that also meets the
> requirements of the author of bdd8b6c98239 without breaking
> the i915 driver.

I'm not a developer and I'm don't known the details of this thread and
the backstory of the regression, but it sounds like that's the approach
that is needed here until someone comes up with a fix for the regression
exposed by bdd8b6c98239.

But if I'm wrong, please tell me.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.

> The i915 driver relies on the memory subsytem
> to provide it with an accurate test for the existence of
> X86_FEATURE_PAT. I think your patch provides that more accurate
> test so that bdd8b6c98239 could be re-applied when your patch is
> committed. Juergen's patch would have to touch bdd8b6c98239
> with new functions that probably have unknown and unintended
> consequences, so I think your approach is also better in that regard.
> As regards your patch, there is just a disagreement about how the
> i915 driver should behave if nopat is set. I agree the i915 driver
> could do a better job handling that case, at least with better error
> logs.
> 
> Chuck
> 
>>
>>> I respond by saying a well-written driver should refuse to honor
>>> the incorrect configuration requested by the user and instead
>>> warn the user that it did not honor the incorrect kernel option.
>>>
>>> I am only presuming what your patch would do on my box based
>>> on what I learned about this problem from my debugging. I can
>>> also test your patch on my box to verify that my understanding of
>>> it is correct.
>>>
>>> I also have not yet verified Juergen's patch will not fix it, but
>>> I am almost certain it will not unless it is expanded so it also
>>> touches i915_gem_object_pin_map() with the fix. I plan to test
>>> his patch, but expanded so it touches that function also.
>>>
>>> I also plan to test your patch with and without nopat and report the
>>> results in the thread where you posted your patch. Hopefully
>>> by tomorrow I will have the results.
>>>
>>> Chuck

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Chuck Zmudzinski]

On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
> On 20.05.22 16:48, Chuck Zmudzinski wrote:
>> On 5/20/2022 10:06 AM, Jan Beulich wrote:
>>> On 20.05.2022 15:33, Chuck Zmudzinski wrote:
>>>> On 5/20/2022 5:41 AM, Jan Beulich wrote:
>>>>> On 20.05.2022 10:30, Chuck Zmudzinski wrote:
>>>>>> On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
>>>>>>> On 5/20/2022 2:05 AM, Jan Beulich wrote:
>>>>>>>> On 20.05.2022 06:43, Chuck Zmudzinski wrote:
>>>>>>>>> On 5/4/22 5:14 AM, Juergen Gross wrote:
>>>>>>>>>> On 04.05.22 10:31, Jan Beulich wrote:
>>>>>>>>>>> On 03.05.2022 15:22, Juergen Gross wrote:
>>>>>>>>>>>
>>>>>>>>>>> ... these uses there are several more. You say nothing on why
>>>>>>>>>>> those want
>>>>>>>>>>> leaving unaltered. When preparing my earlier patch I did
>>>>>>>>>>> inspect them
>>>>>>>>>>> and came to the conclusion that these all would also better
>>>>>>>>>>> observe the
>>>>>>>>>>> adjusted behavior (or else I couldn't have left pat_enabled()
>>>>>>>>>>> as the
>>>>>>>>>>> only predicate). In fact, as said in the description of my
>>>>>>>>>>> earlier
>>>>>>>>>>> patch, in
>>>>>>>>>>> my debugging I did find the use in i915_gem_object_pin_map()
>>>>>>>>>>> to be
>>>>>>>>>>> the
>>>>>>>>>>> problematic one, which you leave alone.
>>>>>>>>>> Oh, I missed that one, sorry.
>>>>>>>>> That is why your patch would not fix my Haswell unless
>>>>>>>>> it also touches i915_gem_object_pin_map() in
>>>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>>>>>>>>>
>>>>>>>>>> I wanted to be rather defensive in my changes, but I agree at
>>>>>>>>>> least
>>>>>>>>>> the
>>>>>>>>>> case in arch_phys_wc_add() might want to be changed, too.
>>>>>>>>> I think your approach needs to be more aggressive so it will fix
>>>>>>>>> all the known false negatives introduced by bdd8b6c98239
>>>>>>>>> such as the one in i915_gem_object_pin_map().
>>>>>>>>>
>>>>>>>>> I looked at Jan's approach and I think it would fix the issue
>>>>>>>>> with my Haswell as long as I don't use the nopat option. I
>>>>>>>>> really don't have a strong opinion on that question, but I
>>>>>>>>> think the nopat option as a Linux kernel option, as opposed
>>>>>>>>> to a hypervisor option, should only affect the kernel, and
>>>>>>>>> if the hypervisor provides the pat feature, then the kernel
>>>>>>>>> should not override that,
>>>>>>>> Hmm, why would the kernel not be allowed to override that? Such
>>>>>>>> an override would affect only the single domain where the
>>>>>>>> kernel runs; other domains could take their own decisions.
>>>>>>>>
>>>>>>>> Also, for the sake of completeness: "nopat" used when running on
>>>>>>>> bare metal has the same bad effect on system boot, so there
>>>>>>>> pretty clearly is an error cleanup issue in the i915 driver. But
>>>>>>>> that's orthogonal, and I expect the maintainers may not even care
>>>>>>>> (but tell us "don't do that then").
>>>>>> Actually I just did a test with the last official Debian kernel
>>>>>> build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
>>>>>> applied. In fact, the nopat option does *not* break the i915 driver
>>>>>> in 5.16. That is, with the nopat option, the i915 driver loads
>>>>>> normally on both the bare metal and on the Xen hypervisor.
>>>>>> That means your presumption (and the presumption of
>>>>>> the author of bdd8b6c98239) that the "nopat" option was
>>>>>> being observed by the i915 driver is incorrect. Setting "nopat"
>>>>>> had no effect on my system with Linux 5.16. So after doing these
>>>>>> tests, I am against the aggressive approach of breaking the i915
>>>>>> driver with the "nopat" option because prior to bdd8b6c98239,
>>>>>> nopat did not break the i915 driver. Why break it now?
>>>>> Because that's, in my understanding, is the purpose of "nopat"
>>>>> (not breaking the driver of course - that's a driver bug -, but
>>>>> having an effect on the driver).
>>>> I wouldn't call it a driver bug, but an incorrect configuration of the
>>>> kernel by the user.  I presume X86_FEATURE_PAT is required by the
>>>> i915 driver
>>> The driver ought to work fine without PAT (and hence without being
>>> able to make WC mappings). It would use UC instead and be slow, but
>>> it ought to work.
>>>
>>>> and therefore the driver should refuse to disable
>>>> it if the user requests to disable it and instead warn the user that
>>>> the driver did not disable the feature, contrary to what the user
>>>> requested with the nopat option.
>>>>
>>>> In any case, my test did not verify that when nopat is set in Linux
>>>> 5.16,
>>>> the thread takes the same code path as when nopat is not set,
>>>> so I am not totally sure that the reason nopat does not break the
>>>> i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
>>>> returns true even when nopat is set. I could test it with a custom
>>>> log message in 5.16 if that is necessary.
>>>>
>>>> Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
>>>> to return true in 5.16 when the user requests nopat?
>>> No, I'm not saying that. It was wrong for this construct to be used
>>> in the driver, which was fixed for 5.17 (and which had caused the
>>> regression I did observe, leading to the patch as a hopefully least
>>> bad option).
>>>
>>>> I think that is
>>>> just permitting a bad configuration to break the driver that a
>>>> well-written operating system should not allow. The i915 driver
>>>> was, in my opinion, correctly ignoring the nopat option in 5.16
>>>> because that option is not compatible with the hardware the
>>>> i915 driver is trying to initialize and setup at boot time. At least
>>>> that is my understanding now, but I will need to test it on 5.16
>>>> to be sure I understand it correctly.
>>>>
>>>> Also, AFAICT, your patch would break the driver when the nopat
>>>> option is set and only fix the regression introduced by bdd8b6c98239
>>>> when nopat is not set on my box, so your patch would
>>>> introduce a regression relative to Linux 5.16 and earlier for the
>>>> case when nopat is set on my box. I think your point would
>>>> be that it is not a regression if it is an incorrect user configuration.
>>> Again no - my view is that there's a separate, pre-existing issue
>>> in the driver which was uncovered by the change. This may be a
>>> perceived regression, but is imo different from a real one.
> Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
> comes to the "no regressions rule". Just took a quick look at quotes
> from Linus
> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
> and found this statement from Linus to back this up:
>
> ```
> One _particularly_ last-minute revert is the top-most commit (ignoring
> the version change itself) done just before the release, and while
> it's very annoying, it's perhaps also instructive.
>
> What's instructive about it is that I reverted a commit that wasn't
> actually buggy. In fact, it was doing exactly what it set out to do,
> and did it very well. In fact it did it _so_ well that the much
> improved IO patterns it caused then ended up revealing a user-visible
> regression due to a real bug in a completely unrelated area.
> ```
>
> He said that here:
> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>
> The situation is of course different here, but similar enough.
>
>> Since it is a regression, I think for now bdd8b6c98239 should
>> be reverted and the fix backported to Linux 5.17 stable until
>> the underlying memory subsystem can provide the i915 driver
>> with an updated test for the PAT feature that also meets the
>> requirements of the author of bdd8b6c98239 without breaking
>> the i915 driver.
> I'm not a developer and I'm don't known the details of this thread and
> the backstory of the regression, but it sounds like that's the approach
> that is needed here until someone comes up with a fix for the regression
> exposed by bdd8b6c98239.
>
> But if I'm wrong, please tell me.

You are mostly right, I think. Reverting bdd8b6c98239 fixes
it. There is another way to fix it, though. The patch proposed
by Jan Beulich also fixes the regression on my system, so as
the person reporting this is a regression, I would also be satisfied
with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
posted his proposed patch here:

https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/

The only reservation I have about Jan's patch is that the commit
message does not clearly explain how the patch changes what
the nopat kernel boot option does. It doesn't affect me because
I don't use nopat, but it should probably be mentioned in the
commit message, as pointed out here:

https://lore.kernel.org/lkml/bd9ed2c2-1337-27bb-c9da-dfc7b31d492c@netscape.net/

Whatever fix for the regression exposed by bdd8b6c98239 also
needs to be backported to the stable versions 5.17 and 5.18.

Regards,

Chuck Zmudzinski
>
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
>
> P.S.: As the Linux kernel's regression tracker I deal with a lot of
> reports and sometimes miss something important when writing mails like
> this. If that's the case here, don't hesitate to tell me in a public
> reply, it's in everyone's interest to set the public record straight.
>
>> The i915 driver relies on the memory subsytem
>> to provide it with an accurate test for the existence of
>> X86_FEATURE_PAT. I think your patch provides that more accurate
>> test so that bdd8b6c98239 could be re-applied when your patch is
>> committed. Juergen's patch would have to touch bdd8b6c98239
>> with new functions that probably have unknown and unintended
>> consequences, so I think your approach is also better in that regard.
>> As regards your patch, there is just a disagreement about how the
>> i915 driver should behave if nopat is set. I agree the i915 driver
>> could do a better job handling that case, at least with better error
>> logs.
>>
>> Chuck
>>
>>>> I respond by saying a well-written driver should refuse to honor
>>>> the incorrect configuration requested by the user and instead
>>>> warn the user that it did not honor the incorrect kernel option.
>>>>
>>>> I am only presuming what your patch would do on my box based
>>>> on what I learned about this problem from my debugging. I can
>>>> also test your patch on my box to verify that my understanding of
>>>> it is correct.
>>>>
>>>> I also have not yet verified Juergen's patch will not fix it, but
>>>> I am almost certain it will not unless it is expanded so it also
>>>> touches i915_gem_object_pin_map() with the fix. I plan to test
>>>> his patch, but expanded so it touches that function also.
>>>>
>>>> I also plan to test your patch with and without nopat and report the
>>>> results in the thread where you posted your patch. Hopefully
>>>> by tomorrow I will have the results.
>>>>
>>>> Chuck

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Thorsten Leemhuis]

On 24.05.22 20:32, Chuck Zmudzinski wrote:
> On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
>> On 20.05.22 16:48, Chuck Zmudzinski wrote:
>>> On 5/20/2022 10:06 AM, Jan Beulich wrote:
>>>> On 20.05.2022 15:33, Chuck Zmudzinski wrote:
>>>>> On 5/20/2022 5:41 AM, Jan Beulich wrote:
>>>>>> On 20.05.2022 10:30, Chuck Zmudzinski wrote:
>>>>>>> On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
>>>>>>>> On 5/20/2022 2:05 AM, Jan Beulich wrote:
>>>>>>>>> On 20.05.2022 06:43, Chuck Zmudzinski wrote:
>>>>>>>>>> On 5/4/22 5:14 AM, Juergen Gross wrote:
>>>>>>>>>>> On 04.05.22 10:31, Jan Beulich wrote:
>>>>>>>>>>>> On 03.05.2022 15:22, Juergen Gross wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> ... these uses there are several more. You say nothing on why
>>>>>>>>>>>> those want
>>>>>>>>>>>> leaving unaltered. When preparing my earlier patch I did
>>>>>>>>>>>> inspect them
>>>>>>>>>>>> and came to the conclusion that these all would also better
>>>>>>>>>>>> observe the
>>>>>>>>>>>> adjusted behavior (or else I couldn't have left pat_enabled()
>>>>>>>>>>>> as the
>>>>>>>>>>>> only predicate). In fact, as said in the description of my
>>>>>>>>>>>> earlier
>>>>>>>>>>>> patch, in
>>>>>>>>>>>> my debugging I did find the use in i915_gem_object_pin_map()
>>>>>>>>>>>> to be
>>>>>>>>>>>> the
>>>>>>>>>>>> problematic one, which you leave alone.
>>>>>>>>>>> Oh, I missed that one, sorry.
>>>>>>>>>> That is why your patch would not fix my Haswell unless
>>>>>>>>>> it also touches i915_gem_object_pin_map() in
>>>>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>>>>>>>>>>
>>>>>>>>>>> I wanted to be rather defensive in my changes, but I agree at
>>>>>>>>>>> least
>>>>>>>>>>> the
>>>>>>>>>>> case in arch_phys_wc_add() might want to be changed, too.
>>>>>>>>>> I think your approach needs to be more aggressive so it will fix
>>>>>>>>>> all the known false negatives introduced by bdd8b6c98239
>>>>>>>>>> such as the one in i915_gem_object_pin_map().
>>>>>>>>>>
>>>>>>>>>> I looked at Jan's approach and I think it would fix the issue
>>>>>>>>>> with my Haswell as long as I don't use the nopat option. I
>>>>>>>>>> really don't have a strong opinion on that question, but I
>>>>>>>>>> think the nopat option as a Linux kernel option, as opposed
>>>>>>>>>> to a hypervisor option, should only affect the kernel, and
>>>>>>>>>> if the hypervisor provides the pat feature, then the kernel
>>>>>>>>>> should not override that,
>>>>>>>>> Hmm, why would the kernel not be allowed to override that? Such
>>>>>>>>> an override would affect only the single domain where the
>>>>>>>>> kernel runs; other domains could take their own decisions.
>>>>>>>>>
>>>>>>>>> Also, for the sake of completeness: "nopat" used when running on
>>>>>>>>> bare metal has the same bad effect on system boot, so there
>>>>>>>>> pretty clearly is an error cleanup issue in the i915 driver. But
>>>>>>>>> that's orthogonal, and I expect the maintainers may not even care
>>>>>>>>> (but tell us "don't do that then").
>>>>>>> Actually I just did a test with the last official Debian kernel
>>>>>>> build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
>>>>>>> applied. In fact, the nopat option does *not* break the i915 driver
>>>>>>> in 5.16. That is, with the nopat option, the i915 driver loads
>>>>>>> normally on both the bare metal and on the Xen hypervisor.
>>>>>>> That means your presumption (and the presumption of
>>>>>>> the author of bdd8b6c98239) that the "nopat" option was
>>>>>>> being observed by the i915 driver is incorrect. Setting "nopat"
>>>>>>> had no effect on my system with Linux 5.16. So after doing these
>>>>>>> tests, I am against the aggressive approach of breaking the i915
>>>>>>> driver with the "nopat" option because prior to bdd8b6c98239,
>>>>>>> nopat did not break the i915 driver. Why break it now?
>>>>>> Because that's, in my understanding, is the purpose of "nopat"
>>>>>> (not breaking the driver of course - that's a driver bug -, but
>>>>>> having an effect on the driver).
>>>>> I wouldn't call it a driver bug, but an incorrect configuration of the
>>>>> kernel by the user.  I presume X86_FEATURE_PAT is required by the
>>>>> i915 driver
>>>> The driver ought to work fine without PAT (and hence without being
>>>> able to make WC mappings). It would use UC instead and be slow, but
>>>> it ought to work.
>>>>
>>>>> and therefore the driver should refuse to disable
>>>>> it if the user requests to disable it and instead warn the user that
>>>>> the driver did not disable the feature, contrary to what the user
>>>>> requested with the nopat option.
>>>>>
>>>>> In any case, my test did not verify that when nopat is set in Linux
>>>>> 5.16,
>>>>> the thread takes the same code path as when nopat is not set,
>>>>> so I am not totally sure that the reason nopat does not break the
>>>>> i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
>>>>> returns true even when nopat is set. I could test it with a custom
>>>>> log message in 5.16 if that is necessary.
>>>>>
>>>>> Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
>>>>> to return true in 5.16 when the user requests nopat?
>>>> No, I'm not saying that. It was wrong for this construct to be used
>>>> in the driver, which was fixed for 5.17 (and which had caused the
>>>> regression I did observe, leading to the patch as a hopefully least
>>>> bad option).
>>>>
>>>>> I think that is
>>>>> just permitting a bad configuration to break the driver that a
>>>>> well-written operating system should not allow. The i915 driver
>>>>> was, in my opinion, correctly ignoring the nopat option in 5.16
>>>>> because that option is not compatible with the hardware the
>>>>> i915 driver is trying to initialize and setup at boot time. At least
>>>>> that is my understanding now, but I will need to test it on 5.16
>>>>> to be sure I understand it correctly.
>>>>>
>>>>> Also, AFAICT, your patch would break the driver when the nopat
>>>>> option is set and only fix the regression introduced by bdd8b6c98239
>>>>> when nopat is not set on my box, so your patch would
>>>>> introduce a regression relative to Linux 5.16 and earlier for the
>>>>> case when nopat is set on my box. I think your point would
>>>>> be that it is not a regression if it is an incorrect user
>>>>> configuration.
>>>> Again no - my view is that there's a separate, pre-existing issue
>>>> in the driver which was uncovered by the change. This may be a
>>>> perceived regression, but is imo different from a real one.
>> Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
>> comes to the "no regressions rule". Just took a quick look at quotes
>> from Linus
>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>> and found this statement from Linus to back this up:
>>
>> ```
>> One _particularly_ last-minute revert is the top-most commit (ignoring
>> the version change itself) done just before the release, and while
>> it's very annoying, it's perhaps also instructive.
>>
>> What's instructive about it is that I reverted a commit that wasn't
>> actually buggy. In fact, it was doing exactly what it set out to do,
>> and did it very well. In fact it did it _so_ well that the much
>> improved IO patterns it caused then ended up revealing a user-visible
>> regression due to a real bug in a completely unrelated area.
>> ```
>>
>> He said that here:
>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>
>> The situation is of course different here, but similar enough.
>>
>>> Since it is a regression, I think for now bdd8b6c98239 should
>>> be reverted and the fix backported to Linux 5.17 stable until
>>> the underlying memory subsystem can provide the i915 driver
>>> with an updated test for the PAT feature that also meets the
>>> requirements of the author of bdd8b6c98239 without breaking
>>> the i915 driver.
>> I'm not a developer and I'm don't known the details of this thread and
>> the backstory of the regression, but it sounds like that's the approach
>> that is needed here until someone comes up with a fix for the regression
>> exposed by bdd8b6c98239.
>>
>> But if I'm wrong, please tell me.
> 
> You are mostly right, I think. Reverting bdd8b6c98239 fixes
> it. There is another way to fix it, though.

Yeah, I'm aware of it. But it seems...

> The patch proposed
> by Jan Beulich also fixes the regression on my system, so as
> the person reporting this is a regression, I would also be satisfied
> with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
> posted his proposed patch here:
> 
> https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/

...that approach is not making any progress either?

Jan, can could provide a short status update here? I'd really like to
get this regression fixed one way or another rather sooner than later,
as this is taken way to long already IMHO.

> The only reservation I have about Jan's patch is that the commit
> message does not clearly explain how the patch changes what
> the nopat kernel boot option does. It doesn't affect me because
> I don't use nopat, but it should probably be mentioned in the
> commit message, as pointed out here:
> 
> https://lore.kernel.org/lkml/bd9ed2c2-1337-27bb-c9da-dfc7b31d492c@netscape.net/
> 
> 
> Whatever fix for the regression exposed by bdd8b6c98239 also
> needs to be backported to the stable versions 5.17 and 5.18.

Sure.

BTW, as you seem to be familiar with the issue: there is another report
about a regression WRT to Xen and i915 (that is also not making really
progress):
https://lore.kernel.org/lkml/Yn%2FTgj1Ehs%2FBdpHp@itl-email/

It's just a wild guess, but bould this somehow be related?

Ciao, Thorsten

>>> The i915 driver relies on the memory subsytem
>>> to provide it with an accurate test for the existence of
>>> X86_FEATURE_PAT. I think your patch provides that more accurate
>>> test so that bdd8b6c98239 could be re-applied when your patch is
>>> committed. Juergen's patch would have to touch bdd8b6c98239
>>> with new functions that probably have unknown and unintended
>>> consequences, so I think your approach is also better in that regard.
>>> As regards your patch, there is just a disagreement about how the
>>> i915 driver should behave if nopat is set. I agree the i915 driver
>>> could do a better job handling that case, at least with better error
>>> logs.
>>>
>>> Chuck
>>>
>>>>> I respond by saying a well-written driver should refuse to honor
>>>>> the incorrect configuration requested by the user and instead
>>>>> warn the user that it did not honor the incorrect kernel option.
>>>>>
>>>>> I am only presuming what your patch would do on my box based
>>>>> on what I learned about this problem from my debugging. I can
>>>>> also test your patch on my box to verify that my understanding of
>>>>> it is correct.
>>>>>
>>>>> I also have not yet verified Juergen's patch will not fix it, but
>>>>> I am almost certain it will not unless it is expanded so it also
>>>>> touches i915_gem_object_pin_map() with the fix. I plan to test
>>>>> his patch, but expanded so it touches that function also.
>>>>>
>>>>> I also plan to test your patch with and without nopat and report the
>>>>> results in the thread where you posted your patch. Hopefully
>>>>> by tomorrow I will have the results.
>>>>>
>>>>> Chuck
> 
> 

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 10268 bytes --]

On 25.05.22 09:45, Thorsten Leemhuis wrote:
> 
> 
> On 24.05.22 20:32, Chuck Zmudzinski wrote:
>> On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
>>> On 20.05.22 16:48, Chuck Zmudzinski wrote:
>>>> On 5/20/2022 10:06 AM, Jan Beulich wrote:
>>>>> On 20.05.2022 15:33, Chuck Zmudzinski wrote:
>>>>>> On 5/20/2022 5:41 AM, Jan Beulich wrote:
>>>>>>> On 20.05.2022 10:30, Chuck Zmudzinski wrote:
>>>>>>>> On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
>>>>>>>>> On 5/20/2022 2:05 AM, Jan Beulich wrote:
>>>>>>>>>> On 20.05.2022 06:43, Chuck Zmudzinski wrote:
>>>>>>>>>>> On 5/4/22 5:14 AM, Juergen Gross wrote:
>>>>>>>>>>>> On 04.05.22 10:31, Jan Beulich wrote:
>>>>>>>>>>>>> On 03.05.2022 15:22, Juergen Gross wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> ... these uses there are several more. You say nothing on why
>>>>>>>>>>>>> those want
>>>>>>>>>>>>> leaving unaltered. When preparing my earlier patch I did
>>>>>>>>>>>>> inspect them
>>>>>>>>>>>>> and came to the conclusion that these all would also better
>>>>>>>>>>>>> observe the
>>>>>>>>>>>>> adjusted behavior (or else I couldn't have left pat_enabled()
>>>>>>>>>>>>> as the
>>>>>>>>>>>>> only predicate). In fact, as said in the description of my
>>>>>>>>>>>>> earlier
>>>>>>>>>>>>> patch, in
>>>>>>>>>>>>> my debugging I did find the use in i915_gem_object_pin_map()
>>>>>>>>>>>>> to be
>>>>>>>>>>>>> the
>>>>>>>>>>>>> problematic one, which you leave alone.
>>>>>>>>>>>> Oh, I missed that one, sorry.
>>>>>>>>>>> That is why your patch would not fix my Haswell unless
>>>>>>>>>>> it also touches i915_gem_object_pin_map() in
>>>>>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>>>>>>>>>>>
>>>>>>>>>>>> I wanted to be rather defensive in my changes, but I agree at
>>>>>>>>>>>> least
>>>>>>>>>>>> the
>>>>>>>>>>>> case in arch_phys_wc_add() might want to be changed, too.
>>>>>>>>>>> I think your approach needs to be more aggressive so it will fix
>>>>>>>>>>> all the known false negatives introduced by bdd8b6c98239
>>>>>>>>>>> such as the one in i915_gem_object_pin_map().
>>>>>>>>>>>
>>>>>>>>>>> I looked at Jan's approach and I think it would fix the issue
>>>>>>>>>>> with my Haswell as long as I don't use the nopat option. I
>>>>>>>>>>> really don't have a strong opinion on that question, but I
>>>>>>>>>>> think the nopat option as a Linux kernel option, as opposed
>>>>>>>>>>> to a hypervisor option, should only affect the kernel, and
>>>>>>>>>>> if the hypervisor provides the pat feature, then the kernel
>>>>>>>>>>> should not override that,
>>>>>>>>>> Hmm, why would the kernel not be allowed to override that? Such
>>>>>>>>>> an override would affect only the single domain where the
>>>>>>>>>> kernel runs; other domains could take their own decisions.
>>>>>>>>>>
>>>>>>>>>> Also, for the sake of completeness: "nopat" used when running on
>>>>>>>>>> bare metal has the same bad effect on system boot, so there
>>>>>>>>>> pretty clearly is an error cleanup issue in the i915 driver. But
>>>>>>>>>> that's orthogonal, and I expect the maintainers may not even care
>>>>>>>>>> (but tell us "don't do that then").
>>>>>>>> Actually I just did a test with the last official Debian kernel
>>>>>>>> build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
>>>>>>>> applied. In fact, the nopat option does *not* break the i915 driver
>>>>>>>> in 5.16. That is, with the nopat option, the i915 driver loads
>>>>>>>> normally on both the bare metal and on the Xen hypervisor.
>>>>>>>> That means your presumption (and the presumption of
>>>>>>>> the author of bdd8b6c98239) that the "nopat" option was
>>>>>>>> being observed by the i915 driver is incorrect. Setting "nopat"
>>>>>>>> had no effect on my system with Linux 5.16. So after doing these
>>>>>>>> tests, I am against the aggressive approach of breaking the i915
>>>>>>>> driver with the "nopat" option because prior to bdd8b6c98239,
>>>>>>>> nopat did not break the i915 driver. Why break it now?
>>>>>>> Because that's, in my understanding, is the purpose of "nopat"
>>>>>>> (not breaking the driver of course - that's a driver bug -, but
>>>>>>> having an effect on the driver).
>>>>>> I wouldn't call it a driver bug, but an incorrect configuration of the
>>>>>> kernel by the user.  I presume X86_FEATURE_PAT is required by the
>>>>>> i915 driver
>>>>> The driver ought to work fine without PAT (and hence without being
>>>>> able to make WC mappings). It would use UC instead and be slow, but
>>>>> it ought to work.
>>>>>
>>>>>> and therefore the driver should refuse to disable
>>>>>> it if the user requests to disable it and instead warn the user that
>>>>>> the driver did not disable the feature, contrary to what the user
>>>>>> requested with the nopat option.
>>>>>>
>>>>>> In any case, my test did not verify that when nopat is set in Linux
>>>>>> 5.16,
>>>>>> the thread takes the same code path as when nopat is not set,
>>>>>> so I am not totally sure that the reason nopat does not break the
>>>>>> i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
>>>>>> returns true even when nopat is set. I could test it with a custom
>>>>>> log message in 5.16 if that is necessary.
>>>>>>
>>>>>> Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
>>>>>> to return true in 5.16 when the user requests nopat?
>>>>> No, I'm not saying that. It was wrong for this construct to be used
>>>>> in the driver, which was fixed for 5.17 (and which had caused the
>>>>> regression I did observe, leading to the patch as a hopefully least
>>>>> bad option).
>>>>>
>>>>>> I think that is
>>>>>> just permitting a bad configuration to break the driver that a
>>>>>> well-written operating system should not allow. The i915 driver
>>>>>> was, in my opinion, correctly ignoring the nopat option in 5.16
>>>>>> because that option is not compatible with the hardware the
>>>>>> i915 driver is trying to initialize and setup at boot time. At least
>>>>>> that is my understanding now, but I will need to test it on 5.16
>>>>>> to be sure I understand it correctly.
>>>>>>
>>>>>> Also, AFAICT, your patch would break the driver when the nopat
>>>>>> option is set and only fix the regression introduced by bdd8b6c98239
>>>>>> when nopat is not set on my box, so your patch would
>>>>>> introduce a regression relative to Linux 5.16 and earlier for the
>>>>>> case when nopat is set on my box. I think your point would
>>>>>> be that it is not a regression if it is an incorrect user
>>>>>> configuration.
>>>>> Again no - my view is that there's a separate, pre-existing issue
>>>>> in the driver which was uncovered by the change. This may be a
>>>>> perceived regression, but is imo different from a real one.
>>> Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
>>> comes to the "no regressions rule". Just took a quick look at quotes
>>> from Linus
>>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>> and found this statement from Linus to back this up:
>>>
>>> ```
>>> One _particularly_ last-minute revert is the top-most commit (ignoring
>>> the version change itself) done just before the release, and while
>>> it's very annoying, it's perhaps also instructive.
>>>
>>> What's instructive about it is that I reverted a commit that wasn't
>>> actually buggy. In fact, it was doing exactly what it set out to do,
>>> and did it very well. In fact it did it _so_ well that the much
>>> improved IO patterns it caused then ended up revealing a user-visible
>>> regression due to a real bug in a completely unrelated area.
>>> ```
>>>
>>> He said that here:
>>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>>
>>> The situation is of course different here, but similar enough.
>>>
>>>> Since it is a regression, I think for now bdd8b6c98239 should
>>>> be reverted and the fix backported to Linux 5.17 stable until
>>>> the underlying memory subsystem can provide the i915 driver
>>>> with an updated test for the PAT feature that also meets the
>>>> requirements of the author of bdd8b6c98239 without breaking
>>>> the i915 driver.
>>> I'm not a developer and I'm don't known the details of this thread and
>>> the backstory of the regression, but it sounds like that's the approach
>>> that is needed here until someone comes up with a fix for the regression
>>> exposed by bdd8b6c98239.
>>>
>>> But if I'm wrong, please tell me.
>>
>> You are mostly right, I think. Reverting bdd8b6c98239 fixes
>> it. There is another way to fix it, though.
> 
> Yeah, I'm aware of it. But it seems...
> 
>> The patch proposed
>> by Jan Beulich also fixes the regression on my system, so as
>> the person reporting this is a regression, I would also be satisfied
>> with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
>> posted his proposed patch here:
>>
>> https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/
> 
> ...that approach is not making any progress either?
> 
> Jan, can could provide a short status update here? I'd really like to
> get this regression fixed one way or another rather sooner than later,
> as this is taken way to long already IMHO.
> 
>> The only reservation I have about Jan's patch is that the commit
>> message does not clearly explain how the patch changes what
>> the nopat kernel boot option does. It doesn't affect me because
>> I don't use nopat, but it should probably be mentioned in the
>> commit message, as pointed out here:
>>
>> https://lore.kernel.org/lkml/bd9ed2c2-1337-27bb-c9da-dfc7b31d492c@netscape.net/
>>
>>
>> Whatever fix for the regression exposed by bdd8b6c98239 also
>> needs to be backported to the stable versions 5.17 and 5.18.
> 
> Sure.
> 
> BTW, as you seem to be familiar with the issue: there is another report
> about a regression WRT to Xen and i915 (that is also not making really
> progress):
> https://lore.kernel.org/lkml/Yn%2FTgj1Ehs%2FBdpHp@itl-email/
> 
> It's just a wild guess, but bould this somehow be related?

No, doesn't seem so.

I'm just reviewing the suggested fix:

https://lore.kernel.org/lkml/Yo0LwmVUDSBZb44K@itl-email/


Juergen

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Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Thorsten Leemhuis]

On 25.05.22 10:37, Jan Beulich wrote:
> On 25.05.2022 09:45, Thorsten Leemhuis wrote:
>> On 24.05.22 20:32, Chuck Zmudzinski wrote:
>>> On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
>>>> I'm not a developer and I'm don't known the details of this thread and
>>>> the backstory of the regression, but it sounds like that's the approach
>>>> that is needed here until someone comes up with a fix for the regression
>>>> exposed by bdd8b6c98239.
>>>>
>>>> But if I'm wrong, please tell me.
>>>
>>> You are mostly right, I think. Reverting bdd8b6c98239 fixes
>>> it. There is another way to fix it, though.
>>
>> Yeah, I'm aware of it. But it seems...
>>
>>> The patch proposed
>>> by Jan Beulich also fixes the regression on my system, so as
>>> the person reporting this is a regression, I would also be satisfied
>>> with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
>>> posted his proposed patch here:
>>>
>>> https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/
>>
>> ...that approach is not making any progress either?
>>
>> Jan, can could provide a short status update here? I'd really like to
>> get this regression fixed one way or another rather sooner than later,
>> as this is taken way to long already IMHO.
> 
> What kind of status update could I provide? I've not heard back from
> anyone of the maintainers, so I have no way to know what (if anything)
> I need to do.

That is perfectly fine as a status update for me (I track a lot of
regression and it's easy to miss updated patches, discussion in other
places, and things like that).

Could you maybe send a reminder to the maintainer that this is a fix for
regression that is bothering people and needs to be handled with high
priority? Feel free to tell them the Linux kernel regression tracker is
pestering you because things are taken so long. :-D

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Chuck Zmudzinski]

On 5/25/2022 3:45 AM, Thorsten Leemhuis wrote:
> On 24.05.22 20:32, Chuck Zmudzinski wrote:
>> On 5/21/22 6:47 AM, Thorsten Leemhuis wrote:
>>> On 20.05.22 16:48, Chuck Zmudzinski wrote:
>>>> On 5/20/2022 10:06 AM, Jan Beulich wrote:
>>>>> On 20.05.2022 15:33, Chuck Zmudzinski wrote:
>>>>>> On 5/20/2022 5:41 AM, Jan Beulich wrote:
>>>>>>> On 20.05.2022 10:30, Chuck Zmudzinski wrote:
>>>>>>>> On 5/20/2022 2:59 AM, Chuck Zmudzinski wrote:
>>>>>>>>> On 5/20/2022 2:05 AM, Jan Beulich wrote:
>>>>>>>>>> On 20.05.2022 06:43, Chuck Zmudzinski wrote:
>>>>>>>>>>> On 5/4/22 5:14 AM, Juergen Gross wrote:
>>>>>>>>>>>> On 04.05.22 10:31, Jan Beulich wrote:
>>>>>>>>>>>>> On 03.05.2022 15:22, Juergen Gross wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> ... these uses there are several more. You say nothing on why
>>>>>>>>>>>>> those want
>>>>>>>>>>>>> leaving unaltered. When preparing my earlier patch I did
>>>>>>>>>>>>> inspect them
>>>>>>>>>>>>> and came to the conclusion that these all would also better
>>>>>>>>>>>>> observe the
>>>>>>>>>>>>> adjusted behavior (or else I couldn't have left pat_enabled()
>>>>>>>>>>>>> as the
>>>>>>>>>>>>> only predicate). In fact, as said in the description of my
>>>>>>>>>>>>> earlier
>>>>>>>>>>>>> patch, in
>>>>>>>>>>>>> my debugging I did find the use in i915_gem_object_pin_map()
>>>>>>>>>>>>> to be
>>>>>>>>>>>>> the
>>>>>>>>>>>>> problematic one, which you leave alone.
>>>>>>>>>>>> Oh, I missed that one, sorry.
>>>>>>>>>>> That is why your patch would not fix my Haswell unless
>>>>>>>>>>> it also touches i915_gem_object_pin_map() in
>>>>>>>>>>> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>>>>>>>>>>>
>>>>>>>>>>>> I wanted to be rather defensive in my changes, but I agree at
>>>>>>>>>>>> least
>>>>>>>>>>>> the
>>>>>>>>>>>> case in arch_phys_wc_add() might want to be changed, too.
>>>>>>>>>>> I think your approach needs to be more aggressive so it will fix
>>>>>>>>>>> all the known false negatives introduced by bdd8b6c98239
>>>>>>>>>>> such as the one in i915_gem_object_pin_map().
>>>>>>>>>>>
>>>>>>>>>>> I looked at Jan's approach and I think it would fix the issue
>>>>>>>>>>> with my Haswell as long as I don't use the nopat option. I
>>>>>>>>>>> really don't have a strong opinion on that question, but I
>>>>>>>>>>> think the nopat option as a Linux kernel option, as opposed
>>>>>>>>>>> to a hypervisor option, should only affect the kernel, and
>>>>>>>>>>> if the hypervisor provides the pat feature, then the kernel
>>>>>>>>>>> should not override that,
>>>>>>>>>> Hmm, why would the kernel not be allowed to override that? Such
>>>>>>>>>> an override would affect only the single domain where the
>>>>>>>>>> kernel runs; other domains could take their own decisions.
>>>>>>>>>>
>>>>>>>>>> Also, for the sake of completeness: "nopat" used when running on
>>>>>>>>>> bare metal has the same bad effect on system boot, so there
>>>>>>>>>> pretty clearly is an error cleanup issue in the i915 driver. But
>>>>>>>>>> that's orthogonal, and I expect the maintainers may not even care
>>>>>>>>>> (but tell us "don't do that then").
>>>>>>>> Actually I just did a test with the last official Debian kernel
>>>>>>>> build of Linux 5.16, that is, a kernel before bdd8b6c98239 was
>>>>>>>> applied. In fact, the nopat option does *not* break the i915 driver
>>>>>>>> in 5.16. That is, with the nopat option, the i915 driver loads
>>>>>>>> normally on both the bare metal and on the Xen hypervisor.
>>>>>>>> That means your presumption (and the presumption of
>>>>>>>> the author of bdd8b6c98239) that the "nopat" option was
>>>>>>>> being observed by the i915 driver is incorrect. Setting "nopat"
>>>>>>>> had no effect on my system with Linux 5.16. So after doing these
>>>>>>>> tests, I am against the aggressive approach of breaking the i915
>>>>>>>> driver with the "nopat" option because prior to bdd8b6c98239,
>>>>>>>> nopat did not break the i915 driver. Why break it now?
>>>>>>> Because that's, in my understanding, is the purpose of "nopat"
>>>>>>> (not breaking the driver of course - that's a driver bug -, but
>>>>>>> having an effect on the driver).
>>>>>> I wouldn't call it a driver bug, but an incorrect configuration of the
>>>>>> kernel by the user.  I presume X86_FEATURE_PAT is required by the
>>>>>> i915 driver
>>>>> The driver ought to work fine without PAT (and hence without being
>>>>> able to make WC mappings). It would use UC instead and be slow, but
>>>>> it ought to work.
>>>>>
>>>>>> and therefore the driver should refuse to disable
>>>>>> it if the user requests to disable it and instead warn the user that
>>>>>> the driver did not disable the feature, contrary to what the user
>>>>>> requested with the nopat option.
>>>>>>
>>>>>> In any case, my test did not verify that when nopat is set in Linux
>>>>>> 5.16,
>>>>>> the thread takes the same code path as when nopat is not set,
>>>>>> so I am not totally sure that the reason nopat does not break the
>>>>>> i915 driver in 5.16 is that static_cpu_has(X86_FEATURE_PAT)
>>>>>> returns true even when nopat is set. I could test it with a custom
>>>>>> log message in 5.16 if that is necessary.
>>>>>>
>>>>>> Are you saying it was wrong for static_cpu_has(X86_FEATURE_PAT)
>>>>>> to return true in 5.16 when the user requests nopat?
>>>>> No, I'm not saying that. It was wrong for this construct to be used
>>>>> in the driver, which was fixed for 5.17 (and which had caused the
>>>>> regression I did observe, leading to the patch as a hopefully least
>>>>> bad option).
>>>>>
>>>>>> I think that is
>>>>>> just permitting a bad configuration to break the driver that a
>>>>>> well-written operating system should not allow. The i915 driver
>>>>>> was, in my opinion, correctly ignoring the nopat option in 5.16
>>>>>> because that option is not compatible with the hardware the
>>>>>> i915 driver is trying to initialize and setup at boot time. At least
>>>>>> that is my understanding now, but I will need to test it on 5.16
>>>>>> to be sure I understand it correctly.
>>>>>>
>>>>>> Also, AFAICT, your patch would break the driver when the nopat
>>>>>> option is set and only fix the regression introduced by bdd8b6c98239
>>>>>> when nopat is not set on my box, so your patch would
>>>>>> introduce a regression relative to Linux 5.16 and earlier for the
>>>>>> case when nopat is set on my box. I think your point would
>>>>>> be that it is not a regression if it is an incorrect user
>>>>>> configuration.
>>>>> Again no - my view is that there's a separate, pre-existing issue
>>>>> in the driver which was uncovered by the change. This may be a
>>>>> perceived regression, but is imo different from a real one.
>>> Sorry, for you maybe, but I'm pretty sure for Linus it's not when it
>>> comes to the "no regressions rule". Just took a quick look at quotes
>>> from Linus
>>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>> and found this statement from Linus to back this up:
>>>
>>> ```
>>> One _particularly_ last-minute revert is the top-most commit (ignoring
>>> the version change itself) done just before the release, and while
>>> it's very annoying, it's perhaps also instructive.
>>>
>>> What's instructive about it is that I reverted a commit that wasn't
>>> actually buggy. In fact, it was doing exactly what it set out to do,
>>> and did it very well. In fact it did it _so_ well that the much
>>> improved IO patterns it caused then ended up revealing a user-visible
>>> regression due to a real bug in a completely unrelated area.
>>> ```
>>>
>>> He said that here:
>>> https://www.kernel.org/doc/html/latest/process/handling-regressions.html
>>>
>>> The situation is of course different here, but similar enough.
>>>
>>>> Since it is a regression, I think for now bdd8b6c98239 should
>>>> be reverted and the fix backported to Linux 5.17 stable until
>>>> the underlying memory subsystem can provide the i915 driver
>>>> with an updated test for the PAT feature that also meets the
>>>> requirements of the author of bdd8b6c98239 without breaking
>>>> the i915 driver.
>>> I'm not a developer and I'm don't known the details of this thread and
>>> the backstory of the regression, but it sounds like that's the approach
>>> that is needed here until someone comes up with a fix for the regression
>>> exposed by bdd8b6c98239.
>>>
>>> But if I'm wrong, please tell me.
>> You are mostly right, I think. Reverting bdd8b6c98239 fixes
>> it. There is another way to fix it, though.
> Yeah, I'm aware of it. But it seems...
>
>> The patch proposed
>> by Jan Beulich also fixes the regression on my system, so as
>> the person reporting this is a regression, I would also be satisfied
>> with Jan's patch instead of reverting bdd8b6c98239 as a fix. Jan
>> posted his proposed patch here:
>>
>> https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/
> ...that approach is not making any progress either?

Jan's approach does fix it on my system. There was some debate
about what the kernel nopat option should do, though. I don't
have a strong opinion on that and would accept Jan's patch
as a fix.

>
> Jan, can could provide a short status update here? I'd really like to
> get this regression fixed one way or another rather sooner than later,
> as this is taken way to long already IMHO.

I hope something is done soon also.

>
>> The only reservation I have about Jan's patch is that the commit
>> message does not clearly explain how the patch changes what
>> the nopat kernel boot option does. It doesn't affect me because
>> I don't use nopat, but it should probably be mentioned in the
>> commit message, as pointed out here:
>>
>> https://lore.kernel.org/lkml/bd9ed2c2-1337-27bb-c9da-dfc7b31d492c@netscape.net/
>>
>>
>> Whatever fix for the regression exposed by bdd8b6c98239 also
>> needs to be backported to the stable versions 5.17 and 5.18.
> Sure.
>
> BTW, as you seem to be familiar with the issue: there is another report
> about a regression WRT to Xen and i915 (that is also not making really
> progress):
> https://lore.kernel.org/lkml/Yn%2FTgj1Ehs%2FBdpHp@itl-email/
>
> It's just a wild guess, but bould this somehow be related?

It could be, but I do not run a GUI in my Xen Dom0, so I have not
seen that issue.

Best regards,

Chuck

Re: [Intel-gfx] [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

[Christoph Hellwig]

On Tue, May 03, 2022 at 03:22:07PM +0200, Juergen Gross wrote:
> Some drivers are using pat_enabled() in order to test availability of
> special caching modes (WC and UC-). This will lead to false negatives
> in case the system was booted e.g. with the "nopat" variant and the
> BIOS did setup the PAT MSR supporting the queried mode, or if the
> system is running as a Xen PV guest.
> 
> Add test functions for those caching modes instead and use them at the
> appropriate places.
> 
> For symmetry reasons export the already existing x86_has_pat_wp() for
> modules, too.

No, we never export unused functionality.