Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR

[Juergen Gross <jgross@suse.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 7193 bytes --]

On 15.07.22 04:19, Chuck Zmudzinski wrote:
> On 7/14/2022 1:40 AM, Juergen Gross wrote:
>> On 13.07.22 03:36, Chuck Zmudzinski wrote:
>>> The commit 99c13b8c8896d7bcb92753bf
>>> ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
>>> incorrectly failed to account for the case in init_cache_modes() when
>>> CPUs do support PAT and falsely reported PAT to be disabled when in
>>> fact PAT is enabled. In some environments, notably in Xen PV domains,
>>> MTRR is disabled but PAT is still enabled, and that is the case
>>> that the aforementioned commit failed to account for.
>>>
>>> As an unfortunate consequnce, the pat_enabled() function currently does
>>> not correctly report that PAT is enabled in such environments. The fix
>>> is implemented in init_cache_modes() by setting pat_bp_enabled to true
>>> in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf
>>> ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed
>>> to account for.
>>>
>>> This approach arranges for pat_enabled() to return true in the Xen PV
>>> environment without undermining the rest of PAT MSR management logic
>>> that considers PAT to be disabled: Specifically, no writes to the PAT
>>> MSR should occur.
>>>
>>> This patch fixes a regression that some users are experiencing with
>>> Linux as a Xen Dom0 driving particular Intel graphics devices by
>>> correctly reporting to the Intel i915 driver that PAT is enabled where
>>> previously it was falsely reporting that PAT is disabled. Some users
>>> are experiencing system hangs in Xen PV Dom0 and all users on Xen PV
>>> Dom0 are experiencing reduced graphics performance because the keying of
>>> the use of WC mappings to pat_enabled() (see arch_can_pci_mmap_wc())
>>> means that in particular graphics frame buffer accesses are quite a bit
>>> less performant than possible without this patch.
>>>
>>> Also, with the current code, in the Xen PV environment, PAT will not be
>>> disabled if the administrator sets the "nopat" boot option. Introduce
>>> a new boolean variable, pat_force_disable, to forcibly disable PAT
>>> when the administrator sets the "nopat" option to override the default
>>> behavior of using the PAT configuration that Xen has provided.
>>>
>>> For the new boolean to live in .init.data, init_cache_modes() also needs
>>> moving to .init.text (where it could/should have lived already before).
>>>
>>> Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
>>> Co-developed-by: Jan Beulich <jbeulich@suse.com>
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
>>> ---
>>> v2: *Add force_pat_disabled variable to fix "nopat" on Xen PV (Jan Beulich)
>>>       *Add the necessary code to incorporate the "nopat" fix
>>>       *void init_cache_modes(void) -> void __init init_cache_modes(void)
>>>       *Add Jan Beulich as Co-developer (Jan has not signed off yet)
>>>       *Expand the commit message to include relevant parts of the commit
>>>        message of Jan Beulich's proposed patch for this problem
>>>       *Fix 'else if ... {' placement and indentation
>>>       *Remove indication the backport to stable branches is only back to 5.17.y
>>>
>>> I think these changes address all the comments on the original patch
>>>
>>> I added Jan Beulich as a Co-developer because Juergen Gross asked me to
>>> include Jan's idea for fixing "nopat" that was missing from the first
>>> version of the patch.
>>>
>>> The patch has been tested, it works as expected with and without nopat
>>> in the Xen PV Dom0 environment. That is, "nopat" causes the system to
>>> exhibit the effects and problems that lack of PAT support causes.
>>>
>>>    arch/x86/mm/pat/memtype.c | 16 ++++++++++++++--
>>>    1 file changed, 14 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
>>> index d5ef64ddd35e..10a37d309d23 100644
>>> --- a/arch/x86/mm/pat/memtype.c
>>> +++ b/arch/x86/mm/pat/memtype.c
>>> @@ -62,6 +62,7 @@
>>>    
>>>    static bool __read_mostly pat_bp_initialized;
>>>    static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);
>>> +static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT);
>>>    static bool __read_mostly pat_bp_enabled;
>>>    static bool __read_mostly pat_cm_initialized;
>>>    
>>> @@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason)
>>>    static int __init nopat(char *str)
>>>    {
>>>    	pat_disable("PAT support disabled via boot option.");
>>> +	pat_force_disabled = true;
>>>    	return 0;
>>>    }
>>>    early_param("nopat", nopat);
>>> @@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat)
>>>    	wrmsrl(MSR_IA32_CR_PAT, pat);
>>>    }
>>>    
>>> -void init_cache_modes(void)
>>> +void __init init_cache_modes(void)
>>>    {
>>>    	u64 pat = 0;
>>>    
>>> @@ -292,7 +294,7 @@ void init_cache_modes(void)
>>>    		rdmsrl(MSR_IA32_CR_PAT, pat);
>>>    	}
>>>    
>>> -	if (!pat) {
>>> +	if (!pat || pat_force_disabled) {
>>
>> Can we just remove this modification and ...
>>
>>>    		/*
>>>    		 * No PAT. Emulate the PAT table that corresponds to the two
>>>    		 * cache bits, PWT (Write Through) and PCD (Cache Disable).
>>> @@ -313,6 +315,16 @@ void init_cache_modes(void)
>>>    		 */
>>>    		pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) |
>>>    		      PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
>>> +	} else if (!pat_bp_enabled) {
>>
>> ... use
>>
>> +	} else if (!pat_bp_enabled && !pat_force_disabled) {
>>
>> here?
>>
>> This will result in the desired outcome in all cases IMO: If PAT wasn't
>> disabled via "nopat" and the PAT MSR has a non-zero value (from BIOS or
>> Hypervisor) and PAT has been disabled implicitly (e.g. due to lack of
>> MTRR), then PAT will be set to "enabled" again.
> 
> With that, you can also completely remove the new Boolean - it
> will be a meaningless variable wasting memory. This will also make

No, it is making a difference with "nopat" having been specified.

In the Xen PV case we will have pat_bp_enabled == false due to the
lack of MTRR. We don't want to set it to true if "nopat" has been
specified on the command line, so pat_force_disabled should not be
true when we are setting pat_bp_enabled to true again.

> my patch more or less do what Jan's patch does - the "nopat" option
> will not cause the situation when the PAT MSR does not match the
> software view. So you are basically proposing just going back to
> my original patch, after fixing the style problems, of course. That
> also would solve the problem of needing Jan's S-o-b. I am inclined,
> however, to wait for a maintainer who has power to actually do the
> commit, to make a comment. Your R-b to my v2 did not have much clout
> with the actual maintainers, as far as I can tell. I am somewhat annoyed
> that it was at your suggestion that my v2 ended up confusing the
> main issue, the regression, with the red herring of the "nopat"
> option.

I'm sorry for that.


Juergen

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3149 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]