[RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

By default all BARs map with VMA access permissions
as pgprot_noncached.

In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
is strongly ordered and allows aligned access.
This type of mapping works for NON-PREFETCHABLE bars
containing EP controller registers.
But it restricts PREFETCHABLE bars from doing
unaligned access.

In CMB NVMe drives PREFETCHABLE bars are required to
map as MT_NORMAL_NC to do unaligned access.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
---
 drivers/vfio/pci/vfio_pci.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index b423a30..eff6b65 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1142,7 +1142,10 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
 	}
 
 	vma->vm_private_data = vdev;
-	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+	if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
+		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+	else
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
 	vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
 
 	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
-- 
2.7.4

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Alex Williamson]

On Fri, 13 Jul 2018 10:26:17 +0530
Srinath Mannam <srinath.mannam@broadcom.com> wrote:

> By default all BARs map with VMA access permissions
> as pgprot_noncached.
> 
> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
> is strongly ordered and allows aligned access.
> This type of mapping works for NON-PREFETCHABLE bars
> containing EP controller registers.
> But it restricts PREFETCHABLE bars from doing
> unaligned access.
> 
> In CMB NVMe drives PREFETCHABLE bars are required to
> map as MT_NORMAL_NC to do unaligned access.
> 
> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
> ---

This has been discussed before:

https://www.spinics.net/lists/kvm/msg156548.html

CC'ing the usual suspects from the previous thread.  I'm not convinced
that the patch here has considered anything other than the ARM64
implications and it's not clear that it considers compatibility with
existing users or devices at all.  Can we guarantee for all devices and
use cases that WC is semantically equivalent and preferable to UC?  If
not then we need to device an extension to the interface that allows
the user to specify WC.  Thanks,

Alex

>  drivers/vfio/pci/vfio_pci.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index b423a30..eff6b65 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1142,7 +1142,10 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>  	}
>  
>  	vma->vm_private_data = vdev;
> -	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +	if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
> +		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> +	else
> +		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  	vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
>  
>  	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

Hi Alex,

On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Fri, 13 Jul 2018 10:26:17 +0530
> Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>
>> By default all BARs map with VMA access permissions
>> as pgprot_noncached.
>>
>> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
>> is strongly ordered and allows aligned access.
>> This type of mapping works for NON-PREFETCHABLE bars
>> containing EP controller registers.
>> But it restricts PREFETCHABLE bars from doing
>> unaligned access.
>>
>> In CMB NVMe drives PREFETCHABLE bars are required to
>> map as MT_NORMAL_NC to do unaligned access.
>>
>> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
>> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
>> ---
>
> This has been discussed before:
>
> https://www.spinics.net/lists/kvm/msg156548.html
Thank you for inputs.. I have gone through the long list of mail chain
discussion.
>
> CC'ing the usual suspects from the previous thread.  I'm not convinced
> that the patch here has considered anything other than the ARM64
> implications and it's not clear that it considers compatibility with
> existing users or devices at all.  Can we guarantee for all devices and
> use cases that WC is semantically equivalent and preferable to UC?  If
> not then we need to device an extension to the interface that allows
> the user to specify WC.  Thanks,
>
To implement with user specified WC flags, many changes need to be done.
Suppose In DPDK, prefetcable BARs map using WC flag, then also same
question comes
that WC may be different for different CPUs.
As per functionality, both WC and PREFETCHABLE are same, like merging writes and
typically WC is uncached.
So, based on prefetchable BARs behavior and usage we need to map bar memory.
Is it right to map prefetchable BARs as strongly ordered, aligned
access and uncached?

Please correct me if my understanding is wrong.

Regards,
Srinath.

> Alex
>
>>  drivers/vfio/pci/vfio_pci.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index b423a30..eff6b65 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
>> @@ -1142,7 +1142,10 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>>       }
>>
>>       vma->vm_private_data = vdev;
>> -     vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>> +     if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
>> +             vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>> +     else
>> +             vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>>       vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
>>
>>       return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
>

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Alex Williamson]

On Thu, 19 Jul 2018 00:05:18 +0530
Srinath Mannam <srinath.mannam@broadcom.com> wrote:

> Hi Alex,
> 
> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
> <alex.williamson@redhat.com> wrote:
> > On Fri, 13 Jul 2018 10:26:17 +0530
> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >  
> >> By default all BARs map with VMA access permissions
> >> as pgprot_noncached.
> >>
> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
> >> is strongly ordered and allows aligned access.
> >> This type of mapping works for NON-PREFETCHABLE bars
> >> containing EP controller registers.
> >> But it restricts PREFETCHABLE bars from doing
> >> unaligned access.
> >>
> >> In CMB NVMe drives PREFETCHABLE bars are required to
> >> map as MT_NORMAL_NC to do unaligned access.
> >>
> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
> >> ---  
> >
> > This has been discussed before:
> >
> > https://www.spinics.net/lists/kvm/msg156548.html  
> Thank you for inputs.. I have gone through the long list of mail chain
> discussion.
> >
> > CC'ing the usual suspects from the previous thread.  I'm not convinced
> > that the patch here has considered anything other than the ARM64
> > implications and it's not clear that it considers compatibility with
> > existing users or devices at all.  Can we guarantee for all devices and
> > use cases that WC is semantically equivalent and preferable to UC?  If
> > not then we need to device an extension to the interface that allows
> > the user to specify WC.  Thanks,
> >  
> To implement with user specified WC flags, many changes need to be done.
> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
> question comes
> that WC may be different for different CPUs.
> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
> typically WC is uncached.
> So, based on prefetchable BARs behavior and usage we need to map bar memory.
> Is it right to map prefetchable BARs as strongly ordered, aligned
> access and uncached?

Is it possible to answer that question generically?  Whether to map a
BAR as UC or WC is generally a question for the driver.  Does the
device handle unaligned accesses?  Does the device need strong memory
ordering?  If this is a driver level question then the driver that
needs to make that decision is the userspace driver.  VFIO is just a
pass-through here and since we don't offer the user a choice of
mappings, we take the safer and more conservative mapping, ie. UC.

You're suggesting that there are many changes to be done if we modify
the vfio interface to expose WC under the user's control rather than
simply transparently impose WC for all mappings, but is that really the
case?  Most devices on most platforms seem to work fine now.  Perhaps WC
is a performance optimization, but this is the first instance I've seen
of it as a functional issue.  Does that suggest that the imposed
alignment on your platform is perhaps unique and the relaxed alignment
should be implemented at the architecture specific memory flags for UC
mappings?  For instance, does x86 require this change for the same
device?  The chance for regressions of other devices on other platforms
seems rather high as proposed. Thanks,

Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

HI Alex,

On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Thu, 19 Jul 2018 00:05:18 +0530
> Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>
>> Hi Alex,
>>
>> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
>> <alex.williamson@redhat.com> wrote:
>> > On Fri, 13 Jul 2018 10:26:17 +0530
>> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >
>> >> By default all BARs map with VMA access permissions
>> >> as pgprot_noncached.
>> >>
>> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
>> >> is strongly ordered and allows aligned access.
>> >> This type of mapping works for NON-PREFETCHABLE bars
>> >> containing EP controller registers.
>> >> But it restricts PREFETCHABLE bars from doing
>> >> unaligned access.
>> >>
>> >> In CMB NVMe drives PREFETCHABLE bars are required to
>> >> map as MT_NORMAL_NC to do unaligned access.
>> >>
>> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
>> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
>> >> ---
>> >
>> > This has been discussed before:
>> >
>> > https://www.spinics.net/lists/kvm/msg156548.html
>> Thank you for inputs.. I have gone through the long list of mail chain
>> discussion.
>> >
>> > CC'ing the usual suspects from the previous thread.  I'm not convinced
>> > that the patch here has considered anything other than the ARM64
>> > implications and it's not clear that it considers compatibility with
>> > existing users or devices at all.  Can we guarantee for all devices and
>> > use cases that WC is semantically equivalent and preferable to UC?  If
>> > not then we need to device an extension to the interface that allows
>> > the user to specify WC.  Thanks,
>> >
>> To implement with user specified WC flags, many changes need to be done.
>> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
>> question comes
>> that WC may be different for different CPUs.
>> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
>> typically WC is uncached.
>> So, based on prefetchable BARs behavior and usage we need to map bar memory.
>> Is it right to map prefetchable BARs as strongly ordered, aligned
>> access and uncached?
>
> Is it possible to answer that question generically?  Whether to map a
> BAR as UC or WC is generally a question for the driver.  Does the
> device handle unaligned accesses?  Does the device need strong memory
> ordering?  If this is a driver level question then the driver that
> needs to make that decision is the userspace driver.  VFIO is just a
> pass-through here and since we don't offer the user a choice of
> mappings, we take the safer and more conservative mapping, ie. UC.
>
Yes, you are right, driver should make the decision based on its requirement.
In my case, user space driver is part of SPDK, so SPDK should request DPDK
and DPDK should request VFIO to map BAR for its choice of mapping.
So to implement this we need code changes in VFIO, DPDK and SPDK.

> You're suggesting that there are many changes to be done if we modify
> the vfio interface to expose WC under the user's control rather than
> simply transparently impose WC for all mappings, but is that really the
> case?  Most devices on most platforms seem to work fine now.  Perhaps WC
> is a performance optimization, but this is the first instance I've seen
> of it as a functional issue.  Does that suggest that the imposed
> alignment on your platform is perhaps unique and the relaxed alignment
> should be implemented at the architecture specific memory flags for UC
> mappings?  For instance, does x86 require this change for the same
> device?  The chance for regressions of other devices on other platforms
> seems rather high as proposed. Thanks,
This issue is not specific to platform or device. this is the requirement of
CMB enabled NVMe cards.
NVMe kernel driver already has support to map CMB bar as WC using
ioremap_wc function.
File: drivers/nvme/host/pci.c
Function: nvme_map_cmb
code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
It means ioremap_wc is working with all platforms and WC map of
perfetchable BARs does not harm.
Same is required in SPDK NVMe driver also, without WC map it may work
in x86 platform, but it does not work in ARM platforms.


Regards,
Srinath.
>
> Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Alex Williamson]

On Thu, 19 Jul 2018 20:17:11 +0530
Srinath Mannam <srinath.mannam@broadcom.com> wrote:

> HI Alex,
> 
> On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
> <alex.williamson@redhat.com> wrote:
> > On Thu, 19 Jul 2018 00:05:18 +0530
> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >  
> >> Hi Alex,
> >>
> >> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
> >> <alex.williamson@redhat.com> wrote:  
> >> > On Fri, 13 Jul 2018 10:26:17 +0530
> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >> >  
> >> >> By default all BARs map with VMA access permissions
> >> >> as pgprot_noncached.
> >> >>
> >> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
> >> >> is strongly ordered and allows aligned access.
> >> >> This type of mapping works for NON-PREFETCHABLE bars
> >> >> containing EP controller registers.
> >> >> But it restricts PREFETCHABLE bars from doing
> >> >> unaligned access.
> >> >>
> >> >> In CMB NVMe drives PREFETCHABLE bars are required to
> >> >> map as MT_NORMAL_NC to do unaligned access.
> >> >>
> >> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
> >> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
> >> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
> >> >> ---  
> >> >
> >> > This has been discussed before:
> >> >
> >> > https://www.spinics.net/lists/kvm/msg156548.html  
> >> Thank you for inputs.. I have gone through the long list of mail chain
> >> discussion.  
> >> >
> >> > CC'ing the usual suspects from the previous thread.  I'm not convinced
> >> > that the patch here has considered anything other than the ARM64
> >> > implications and it's not clear that it considers compatibility with
> >> > existing users or devices at all.  Can we guarantee for all devices and
> >> > use cases that WC is semantically equivalent and preferable to UC?  If
> >> > not then we need to device an extension to the interface that allows
> >> > the user to specify WC.  Thanks,
> >> >  
> >> To implement with user specified WC flags, many changes need to be done.
> >> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
> >> question comes
> >> that WC may be different for different CPUs.
> >> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
> >> typically WC is uncached.
> >> So, based on prefetchable BARs behavior and usage we need to map bar memory.
> >> Is it right to map prefetchable BARs as strongly ordered, aligned
> >> access and uncached?  
> >
> > Is it possible to answer that question generically?  Whether to map a
> > BAR as UC or WC is generally a question for the driver.  Does the
> > device handle unaligned accesses?  Does the device need strong memory
> > ordering?  If this is a driver level question then the driver that
> > needs to make that decision is the userspace driver.  VFIO is just a
> > pass-through here and since we don't offer the user a choice of
> > mappings, we take the safer and more conservative mapping, ie. UC.
> >  
> Yes, you are right, driver should make the decision based on its requirement.
> In my case, user space driver is part of SPDK, so SPDK should request DPDK
> and DPDK should request VFIO to map BAR for its choice of mapping.
> So to implement this we need code changes in VFIO, DPDK and SPDK.
> 
> > You're suggesting that there are many changes to be done if we modify
> > the vfio interface to expose WC under the user's control rather than
> > simply transparently impose WC for all mappings, but is that really the
> > case?  Most devices on most platforms seem to work fine now.  Perhaps WC
> > is a performance optimization, but this is the first instance I've seen
> > of it as a functional issue.  Does that suggest that the imposed
> > alignment on your platform is perhaps unique and the relaxed alignment
> > should be implemented at the architecture specific memory flags for UC
> > mappings?  For instance, does x86 require this change for the same
> > device?  The chance for regressions of other devices on other platforms
> > seems rather high as proposed. Thanks,  
> This issue is not specific to platform or device. this is the requirement of
> CMB enabled NVMe cards.
> NVMe kernel driver already has support to map CMB bar as WC using
> ioremap_wc function.
> File: drivers/nvme/host/pci.c
> Function: nvme_map_cmb
> code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
> It means ioremap_wc is working with all platforms and WC map of
> perfetchable BARs does not harm.
> Same is required in SPDK NVMe driver also, without WC map it may work
> in x86 platform, but it does not work in ARM platforms.

Doesn't this contradict your assertion that it's not specific to
platform or device?  The device requires support for unaligned
accesses.  The platform chooses to restrict unaligned accesses for
non-WC mappings while other platforms do not.  The native driver can
still clearly have performance considerations for choosing to use WC
mappings, but it's still not clear to me that the functionality issue
isn't self inflicted by the platform definition of UC vs WC.  Thanks,

Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

HI Alex,

On Thu, Jul 19, 2018 at 8:42 PM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Thu, 19 Jul 2018 20:17:11 +0530
> Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>
>> HI Alex,
>>
>> On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
>> <alex.williamson@redhat.com> wrote:
>> > On Thu, 19 Jul 2018 00:05:18 +0530
>> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >
>> >> Hi Alex,
>> >>
>> >> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
>> >> <alex.williamson@redhat.com> wrote:
>> >> > On Fri, 13 Jul 2018 10:26:17 +0530
>> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >> >
>> >> >> By default all BARs map with VMA access permissions
>> >> >> as pgprot_noncached.
>> >> >>
>> >> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
>> >> >> is strongly ordered and allows aligned access.
>> >> >> This type of mapping works for NON-PREFETCHABLE bars
>> >> >> containing EP controller registers.
>> >> >> But it restricts PREFETCHABLE bars from doing
>> >> >> unaligned access.
>> >> >>
>> >> >> In CMB NVMe drives PREFETCHABLE bars are required to
>> >> >> map as MT_NORMAL_NC to do unaligned access.
>> >> >>
>> >> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> >> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
>> >> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
>> >> >> ---
>> >> >
>> >> > This has been discussed before:
>> >> >
>> >> > https://www.spinics.net/lists/kvm/msg156548.html
>> >> Thank you for inputs.. I have gone through the long list of mail chain
>> >> discussion.
>> >> >
>> >> > CC'ing the usual suspects from the previous thread.  I'm not convinced
>> >> > that the patch here has considered anything other than the ARM64
>> >> > implications and it's not clear that it considers compatibility with
>> >> > existing users or devices at all.  Can we guarantee for all devices and
>> >> > use cases that WC is semantically equivalent and preferable to UC?  If
>> >> > not then we need to device an extension to the interface that allows
>> >> > the user to specify WC.  Thanks,
>> >> >
>> >> To implement with user specified WC flags, many changes need to be done.
>> >> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
>> >> question comes
>> >> that WC may be different for different CPUs.
>> >> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
>> >> typically WC is uncached.
>> >> So, based on prefetchable BARs behavior and usage we need to map bar memory.
>> >> Is it right to map prefetchable BARs as strongly ordered, aligned
>> >> access and uncached?
>> >
>> > Is it possible to answer that question generically?  Whether to map a
>> > BAR as UC or WC is generally a question for the driver.  Does the
>> > device handle unaligned accesses?  Does the device need strong memory
>> > ordering?  If this is a driver level question then the driver that
>> > needs to make that decision is the userspace driver.  VFIO is just a
>> > pass-through here and since we don't offer the user a choice of
>> > mappings, we take the safer and more conservative mapping, ie. UC.
>> >
>> Yes, you are right, driver should make the decision based on its requirement.
>> In my case, user space driver is part of SPDK, so SPDK should request DPDK
>> and DPDK should request VFIO to map BAR for its choice of mapping.
>> So to implement this we need code changes in VFIO, DPDK and SPDK.
>>
>> > You're suggesting that there are many changes to be done if we modify
>> > the vfio interface to expose WC under the user's control rather than
>> > simply transparently impose WC for all mappings, but is that really the
>> > case?  Most devices on most platforms seem to work fine now.  Perhaps WC
>> > is a performance optimization, but this is the first instance I've seen
>> > of it as a functional issue.  Does that suggest that the imposed
>> > alignment on your platform is perhaps unique and the relaxed alignment
>> > should be implemented at the architecture specific memory flags for UC
>> > mappings?  For instance, does x86 require this change for the same
>> > device?  The chance for regressions of other devices on other platforms
>> > seems rather high as proposed. Thanks,
>> This issue is not specific to platform or device. this is the requirement of
>> CMB enabled NVMe cards.
>> NVMe kernel driver already has support to map CMB bar as WC using
>> ioremap_wc function.
>> File: drivers/nvme/host/pci.c
>> Function: nvme_map_cmb
>> code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
>> It means ioremap_wc is working with all platforms and WC map of
>> perfetchable BARs does not harm.
>> Same is required in SPDK NVMe driver also, without WC map it may work
>> in x86 platform, but it does not work in ARM platforms.
>
> Doesn't this contradict your assertion that it's not specific to
> platform or device?  The device requires support for unaligned
> accesses.  The platform chooses to restrict unaligned accesses for
> non-WC mappings while other platforms do not.  The native driver can
> still clearly have performance considerations for choosing to use WC
> mappings, but it's still not clear to me that the functionality issue
> isn't self inflicted by the platform definition of UC vs WC.  Thanks,
>
Device allows both aligned and unaligned access.. so software have
flexibility can do unaligned access.
As per ARM64 platform, with UC map, memory access should be un-cached,
aligned access and strongly order mapping.
with WC map, memory access can be aligned/unaligned and un-cached. I
thought this is the property of platform not issue.
To allow software to do unaligned access of device memory, we need to
use WC map of ARM64 platform case.
In ARM platforms UC mapping is used to map controller registers which
are 4 byte aligned exposed by non-prefetchable bars.
Also prefetchable BARs allows write merging so I thought using WC map
fulfills both write merging (add performance) and unaligned
access.
Can I modify the code as below to enable only for ARM platforms.
        vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+#if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
+       if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
+               vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+#endif
        vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
Regards,
Srinath.

> Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Alex Williamson]

On Thu, 19 Jul 2018 21:49:48 +0530
Srinath Mannam <srinath.mannam@broadcom.com> wrote:

> HI Alex,
> 
> On Thu, Jul 19, 2018 at 8:42 PM, Alex Williamson
> <alex.williamson@redhat.com> wrote:
> > On Thu, 19 Jul 2018 20:17:11 +0530
> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >  
> >> HI Alex,
> >>
> >> On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
> >> <alex.williamson@redhat.com> wrote:  
> >> > On Thu, 19 Jul 2018 00:05:18 +0530
> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >> >  
> >> >> Hi Alex,
> >> >>
> >> >> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
> >> >> <alex.williamson@redhat.com> wrote:  
> >> >> > On Fri, 13 Jul 2018 10:26:17 +0530
> >> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
> >> >> >  
> >> >> >> By default all BARs map with VMA access permissions
> >> >> >> as pgprot_noncached.
> >> >> >>
> >> >> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
> >> >> >> is strongly ordered and allows aligned access.
> >> >> >> This type of mapping works for NON-PREFETCHABLE bars
> >> >> >> containing EP controller registers.
> >> >> >> But it restricts PREFETCHABLE bars from doing
> >> >> >> unaligned access.
> >> >> >>
> >> >> >> In CMB NVMe drives PREFETCHABLE bars are required to
> >> >> >> map as MT_NORMAL_NC to do unaligned access.
> >> >> >>
> >> >> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
> >> >> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
> >> >> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
> >> >> >> ---  
> >> >> >
> >> >> > This has been discussed before:
> >> >> >
> >> >> > https://www.spinics.net/lists/kvm/msg156548.html  
> >> >> Thank you for inputs.. I have gone through the long list of mail chain
> >> >> discussion.  
> >> >> >
> >> >> > CC'ing the usual suspects from the previous thread.  I'm not convinced
> >> >> > that the patch here has considered anything other than the ARM64
> >> >> > implications and it's not clear that it considers compatibility with
> >> >> > existing users or devices at all.  Can we guarantee for all devices and
> >> >> > use cases that WC is semantically equivalent and preferable to UC?  If
> >> >> > not then we need to device an extension to the interface that allows
> >> >> > the user to specify WC.  Thanks,
> >> >> >  
> >> >> To implement with user specified WC flags, many changes need to be done.
> >> >> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
> >> >> question comes
> >> >> that WC may be different for different CPUs.
> >> >> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
> >> >> typically WC is uncached.
> >> >> So, based on prefetchable BARs behavior and usage we need to map bar memory.
> >> >> Is it right to map prefetchable BARs as strongly ordered, aligned
> >> >> access and uncached?  
> >> >
> >> > Is it possible to answer that question generically?  Whether to map a
> >> > BAR as UC or WC is generally a question for the driver.  Does the
> >> > device handle unaligned accesses?  Does the device need strong memory
> >> > ordering?  If this is a driver level question then the driver that
> >> > needs to make that decision is the userspace driver.  VFIO is just a
> >> > pass-through here and since we don't offer the user a choice of
> >> > mappings, we take the safer and more conservative mapping, ie. UC.
> >> >  
> >> Yes, you are right, driver should make the decision based on its requirement.
> >> In my case, user space driver is part of SPDK, so SPDK should request DPDK
> >> and DPDK should request VFIO to map BAR for its choice of mapping.
> >> So to implement this we need code changes in VFIO, DPDK and SPDK.
> >>  
> >> > You're suggesting that there are many changes to be done if we modify
> >> > the vfio interface to expose WC under the user's control rather than
> >> > simply transparently impose WC for all mappings, but is that really the
> >> > case?  Most devices on most platforms seem to work fine now.  Perhaps WC
> >> > is a performance optimization, but this is the first instance I've seen
> >> > of it as a functional issue.  Does that suggest that the imposed
> >> > alignment on your platform is perhaps unique and the relaxed alignment
> >> > should be implemented at the architecture specific memory flags for UC
> >> > mappings?  For instance, does x86 require this change for the same
> >> > device?  The chance for regressions of other devices on other platforms
> >> > seems rather high as proposed. Thanks,  
> >> This issue is not specific to platform or device. this is the requirement of
> >> CMB enabled NVMe cards.
> >> NVMe kernel driver already has support to map CMB bar as WC using
> >> ioremap_wc function.
> >> File: drivers/nvme/host/pci.c
> >> Function: nvme_map_cmb
> >> code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
> >> It means ioremap_wc is working with all platforms and WC map of
> >> perfetchable BARs does not harm.
> >> Same is required in SPDK NVMe driver also, without WC map it may work
> >> in x86 platform, but it does not work in ARM platforms.  
> >
> > Doesn't this contradict your assertion that it's not specific to
> > platform or device?  The device requires support for unaligned
> > accesses.  The platform chooses to restrict unaligned accesses for
> > non-WC mappings while other platforms do not.  The native driver can
> > still clearly have performance considerations for choosing to use WC
> > mappings, but it's still not clear to me that the functionality issue
> > isn't self inflicted by the platform definition of UC vs WC.  Thanks,
> >  
> Device allows both aligned and unaligned access.. so software have
> flexibility can do unaligned access.
> As per ARM64 platform, with UC map, memory access should be un-cached,
> aligned access and strongly order mapping.
> with WC map, memory access can be aligned/unaligned and un-cached. I
> thought this is the property of platform not issue.
> To allow software to do unaligned access of device memory, we need to
> use WC map of ARM64 platform case.
> In ARM platforms UC mapping is used to map controller registers which
> are 4 byte aligned exposed by non-prefetchable bars.
> Also prefetchable BARs allows write merging so I thought using WC map
> fulfills both write merging (add performance) and unaligned
> access.
> Can I modify the code as below to enable only for ARM platforms.
>         vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +#if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
> +       if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
> +               vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> +#endif
>         vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;

While the risk of regression is smaller by restricting this to ARM, I
don't think it's the right solution.  What happens when a device
requires strict ordering?  ARM now behaves differently than any other
architecture, that's not acceptable.  Thanks,

Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

On Sat, Jul 21, 2018 at 1:57 AM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Thu, 19 Jul 2018 21:49:48 +0530
> Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>
>> HI Alex,
>>
>> On Thu, Jul 19, 2018 at 8:42 PM, Alex Williamson
>> <alex.williamson@redhat.com> wrote:
>> > On Thu, 19 Jul 2018 20:17:11 +0530
>> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >
>> >> HI Alex,
>> >>
>> >> On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
>> >> <alex.williamson@redhat.com> wrote:
>> >> > On Thu, 19 Jul 2018 00:05:18 +0530
>> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >> >
>> >> >> Hi Alex,
>> >> >>
>> >> >> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
>> >> >> <alex.williamson@redhat.com> wrote:
>> >> >> > On Fri, 13 Jul 2018 10:26:17 +0530
>> >> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>> >> >> >
>> >> >> >> By default all BARs map with VMA access permissions
>> >> >> >> as pgprot_noncached.
>> >> >> >>
>> >> >> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
>> >> >> >> is strongly ordered and allows aligned access.
>> >> >> >> This type of mapping works for NON-PREFETCHABLE bars
>> >> >> >> containing EP controller registers.
>> >> >> >> But it restricts PREFETCHABLE bars from doing
>> >> >> >> unaligned access.
>> >> >> >>
>> >> >> >> In CMB NVMe drives PREFETCHABLE bars are required to
>> >> >> >> map as MT_NORMAL_NC to do unaligned access.
>> >> >> >>
>> >> >> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> >> >> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
>> >> >> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
>> >> >> >> ---
>> >> >> >
>> >> >> > This has been discussed before:
>> >> >> >
>> >> >> > https://www.spinics.net/lists/kvm/msg156548.html
>> >> >> Thank you for inputs.. I have gone through the long list of mail chain
>> >> >> discussion.
>> >> >> >
>> >> >> > CC'ing the usual suspects from the previous thread.  I'm not convinced
>> >> >> > that the patch here has considered anything other than the ARM64
>> >> >> > implications and it's not clear that it considers compatibility with
>> >> >> > existing users or devices at all.  Can we guarantee for all devices and
>> >> >> > use cases that WC is semantically equivalent and preferable to UC?  If
>> >> >> > not then we need to device an extension to the interface that allows
>> >> >> > the user to specify WC.  Thanks,
>> >> >> >
>> >> >> To implement with user specified WC flags, many changes need to be done.
>> >> >> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
>> >> >> question comes
>> >> >> that WC may be different for different CPUs.
>> >> >> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
>> >> >> typically WC is uncached.
>> >> >> So, based on prefetchable BARs behavior and usage we need to map bar memory.
>> >> >> Is it right to map prefetchable BARs as strongly ordered, aligned
>> >> >> access and uncached?
>> >> >
>> >> > Is it possible to answer that question generically?  Whether to map a
>> >> > BAR as UC or WC is generally a question for the driver.  Does the
>> >> > device handle unaligned accesses?  Does the device need strong memory
>> >> > ordering?  If this is a driver level question then the driver that
>> >> > needs to make that decision is the userspace driver.  VFIO is just a
>> >> > pass-through here and since we don't offer the user a choice of
>> >> > mappings, we take the safer and more conservative mapping, ie. UC.
>> >> >
>> >> Yes, you are right, driver should make the decision based on its requirement.
>> >> In my case, user space driver is part of SPDK, so SPDK should request DPDK
>> >> and DPDK should request VFIO to map BAR for its choice of mapping.
>> >> So to implement this we need code changes in VFIO, DPDK and SPDK.
>> >>
>> >> > You're suggesting that there are many changes to be done if we modify
>> >> > the vfio interface to expose WC under the user's control rather than
>> >> > simply transparently impose WC for all mappings, but is that really the
>> >> > case?  Most devices on most platforms seem to work fine now.  Perhaps WC
>> >> > is a performance optimization, but this is the first instance I've seen
>> >> > of it as a functional issue.  Does that suggest that the imposed
>> >> > alignment on your platform is perhaps unique and the relaxed alignment
>> >> > should be implemented at the architecture specific memory flags for UC
>> >> > mappings?  For instance, does x86 require this change for the same
>> >> > device?  The chance for regressions of other devices on other platforms
>> >> > seems rather high as proposed. Thanks,
>> >> This issue is not specific to platform or device. this is the requirement of
>> >> CMB enabled NVMe cards.
>> >> NVMe kernel driver already has support to map CMB bar as WC using
>> >> ioremap_wc function.
>> >> File: drivers/nvme/host/pci.c
>> >> Function: nvme_map_cmb
>> >> code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
>> >> It means ioremap_wc is working with all platforms and WC map of
>> >> perfetchable BARs does not harm.
>> >> Same is required in SPDK NVMe driver also, without WC map it may work
>> >> in x86 platform, but it does not work in ARM platforms.
>> >
>> > Doesn't this contradict your assertion that it's not specific to
>> > platform or device?  The device requires support for unaligned
>> > accesses.  The platform chooses to restrict unaligned accesses for
>> > non-WC mappings while other platforms do not.  The native driver can
>> > still clearly have performance considerations for choosing to use WC
>> > mappings, but it's still not clear to me that the functionality issue
>> > isn't self inflicted by the platform definition of UC vs WC.  Thanks,
>> >
>> Device allows both aligned and unaligned access.. so software have
>> flexibility can do unaligned access.
>> As per ARM64 platform, with UC map, memory access should be un-cached,
>> aligned access and strongly order mapping.
>> with WC map, memory access can be aligned/unaligned and un-cached. I
>> thought this is the property of platform not issue.
>> To allow software to do unaligned access of device memory, we need to
>> use WC map of ARM64 platform case.
>> In ARM platforms UC mapping is used to map controller registers which
>> are 4 byte aligned exposed by non-prefetchable bars.
>> Also prefetchable BARs allows write merging so I thought using WC map
>> fulfills both write merging (add performance) and unaligned
>> access.
>> Can I modify the code as below to enable only for ARM platforms.
>>         vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>> +#if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
>> +       if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
>> +               vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>> +#endif
>>         vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
>
> While the risk of regression is smaller by restricting this to ARM, I
> don't think it's the right solution.  What happens when a device
> requires strict ordering?  ARM now behaves differently than any other
> architecture, that's not acceptable.  Thanks,
If strict ordering is required for prefetchable bars, driver software
has to add barrier instructions.
With this we can assume for prefetchable bars WC mapping should be fine?

Regards,
Srinath.
>
> Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Srinath Mannam]

Hi Alex,

In user space UIO driver (DPDK) implementation, sysfs interface
"/sys/devices/pci/.../resource0_wc" is used to map prefetchable PCI
resources as WC.
Platforms which support write-combining maps of PCI resources have
arch_can_pci_mmap_wc() flag enabled. So that it allows to map resources as WC.
In this approach mmap calls "pci_mmap_resource_range" kernel function
with write_combine parameter set.
"drivers/pci/pci-sysfs.c" kernel file has this implementation.

If this approach fits to vfio driver, then code change in vfio driver are

 if (arch_can_pci_mmap_wc() &&
     (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH))
            vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
        else
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);

Please provide your feedback.

Thank you.

On Mon, Jul 23, 2018 at 2:03 PM, Srinath Mannam
<srinath.mannam@broadcom.com> wrote:
> On Sat, Jul 21, 2018 at 1:57 AM, Alex Williamson
> <alex.williamson@redhat.com> wrote:
>> On Thu, 19 Jul 2018 21:49:48 +0530
>> Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>>
>>> HI Alex,
>>>
>>> On Thu, Jul 19, 2018 at 8:42 PM, Alex Williamson
>>> <alex.williamson@redhat.com> wrote:
>>> > On Thu, 19 Jul 2018 20:17:11 +0530
>>> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>>> >
>>> >> HI Alex,
>>> >>
>>> >> On Thu, Jul 19, 2018 at 2:55 AM, Alex Williamson
>>> >> <alex.williamson@redhat.com> wrote:
>>> >> > On Thu, 19 Jul 2018 00:05:18 +0530
>>> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>>> >> >
>>> >> >> Hi Alex,
>>> >> >>
>>> >> >> On Tue, Jul 17, 2018 at 8:52 PM, Alex Williamson
>>> >> >> <alex.williamson@redhat.com> wrote:
>>> >> >> > On Fri, 13 Jul 2018 10:26:17 +0530
>>> >> >> > Srinath Mannam <srinath.mannam@broadcom.com> wrote:
>>> >> >> >
>>> >> >> >> By default all BARs map with VMA access permissions
>>> >> >> >> as pgprot_noncached.
>>> >> >> >>
>>> >> >> >> In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
>>> >> >> >> is strongly ordered and allows aligned access.
>>> >> >> >> This type of mapping works for NON-PREFETCHABLE bars
>>> >> >> >> containing EP controller registers.
>>> >> >> >> But it restricts PREFETCHABLE bars from doing
>>> >> >> >> unaligned access.
>>> >> >> >>
>>> >> >> >> In CMB NVMe drives PREFETCHABLE bars are required to
>>> >> >> >> map as MT_NORMAL_NC to do unaligned access.
>>> >> >> >>
>>> >> >> >> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>>> >> >> >> Reviewed-by: Ray Jui <ray.jui@broadcom.com>
>>> >> >> >> Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
>>> >> >> >> ---
>>> >> >> >
>>> >> >> > This has been discussed before:
>>> >> >> >
>>> >> >> > https://www.spinics.net/lists/kvm/msg156548.html
>>> >> >> Thank you for inputs.. I have gone through the long list of mail chain
>>> >> >> discussion.
>>> >> >> >
>>> >> >> > CC'ing the usual suspects from the previous thread.  I'm not convinced
>>> >> >> > that the patch here has considered anything other than the ARM64
>>> >> >> > implications and it's not clear that it considers compatibility with
>>> >> >> > existing users or devices at all.  Can we guarantee for all devices and
>>> >> >> > use cases that WC is semantically equivalent and preferable to UC?  If
>>> >> >> > not then we need to device an extension to the interface that allows
>>> >> >> > the user to specify WC.  Thanks,
>>> >> >> >
>>> >> >> To implement with user specified WC flags, many changes need to be done.
>>> >> >> Suppose In DPDK, prefetcable BARs map using WC flag, then also same
>>> >> >> question comes
>>> >> >> that WC may be different for different CPUs.
>>> >> >> As per functionality, both WC and PREFETCHABLE are same, like merging writes and
>>> >> >> typically WC is uncached.
>>> >> >> So, based on prefetchable BARs behavior and usage we need to map bar memory.
>>> >> >> Is it right to map prefetchable BARs as strongly ordered, aligned
>>> >> >> access and uncached?
>>> >> >
>>> >> > Is it possible to answer that question generically?  Whether to map a
>>> >> > BAR as UC or WC is generally a question for the driver.  Does the
>>> >> > device handle unaligned accesses?  Does the device need strong memory
>>> >> > ordering?  If this is a driver level question then the driver that
>>> >> > needs to make that decision is the userspace driver.  VFIO is just a
>>> >> > pass-through here and since we don't offer the user a choice of
>>> >> > mappings, we take the safer and more conservative mapping, ie. UC.
>>> >> >
>>> >> Yes, you are right, driver should make the decision based on its requirement.
>>> >> In my case, user space driver is part of SPDK, so SPDK should request DPDK
>>> >> and DPDK should request VFIO to map BAR for its choice of mapping.
>>> >> So to implement this we need code changes in VFIO, DPDK and SPDK.
>>> >>
>>> >> > You're suggesting that there are many changes to be done if we modify
>>> >> > the vfio interface to expose WC under the user's control rather than
>>> >> > simply transparently impose WC for all mappings, but is that really the
>>> >> > case?  Most devices on most platforms seem to work fine now.  Perhaps WC
>>> >> > is a performance optimization, but this is the first instance I've seen
>>> >> > of it as a functional issue.  Does that suggest that the imposed
>>> >> > alignment on your platform is perhaps unique and the relaxed alignment
>>> >> > should be implemented at the architecture specific memory flags for UC
>>> >> > mappings?  For instance, does x86 require this change for the same
>>> >> > device?  The chance for regressions of other devices on other platforms
>>> >> > seems rather high as proposed. Thanks,
>>> >> This issue is not specific to platform or device. this is the requirement of
>>> >> CMB enabled NVMe cards.
>>> >> NVMe kernel driver already has support to map CMB bar as WC using
>>> >> ioremap_wc function.
>>> >> File: drivers/nvme/host/pci.c
>>> >> Function: nvme_map_cmb
>>> >> code: dev->cmb = ioremap_wc(pci_resource_start(pdev, bar) + offset, size);
>>> >> It means ioremap_wc is working with all platforms and WC map of
>>> >> perfetchable BARs does not harm.
>>> >> Same is required in SPDK NVMe driver also, without WC map it may work
>>> >> in x86 platform, but it does not work in ARM platforms.
>>> >
>>> > Doesn't this contradict your assertion that it's not specific to
>>> > platform or device?  The device requires support for unaligned
>>> > accesses.  The platform chooses to restrict unaligned accesses for
>>> > non-WC mappings while other platforms do not.  The native driver can
>>> > still clearly have performance considerations for choosing to use WC
>>> > mappings, but it's still not clear to me that the functionality issue
>>> > isn't self inflicted by the platform definition of UC vs WC.  Thanks,
>>> >
>>> Device allows both aligned and unaligned access.. so software have
>>> flexibility can do unaligned access.
>>> As per ARM64 platform, with UC map, memory access should be un-cached,
>>> aligned access and strongly order mapping.
>>> with WC map, memory access can be aligned/unaligned and un-cached. I
>>> thought this is the property of platform not issue.
>>> To allow software to do unaligned access of device memory, we need to
>>> use WC map of ARM64 platform case.
>>> In ARM platforms UC mapping is used to map controller registers which
>>> are 4 byte aligned exposed by non-prefetchable bars.
>>> Also prefetchable BARs allows write merging so I thought using WC map
>>> fulfills both write merging (add performance) and unaligned
>>> access.
>>> Can I modify the code as below to enable only for ARM platforms.
>>>         vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>>> +#if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
>>> +       if (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH)
>>> +               vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>>> +#endif
>>>         vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
>>
>> While the risk of regression is smaller by restricting this to ARM, I
>> don't think it's the right solution.  What happens when a device
>> requires strict ordering?  ARM now behaves differently than any other
>> architecture, that's not acceptable.  Thanks,
> If strict ordering is required for prefetchable bars, driver software
> has to add barrier instructions.
> With this we can assume for prefetchable bars WC mapping should be fine?
>
> Regards,
> Srinath.
>>
>> Alex

Re: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine

[Alex Williamson]

On Wed, 1 Aug 2018 23:28:53 +0530
Srinath Mannam <srinath.mannam@broadcom.com> wrote:

> Hi Alex,
> 
> In user space UIO driver (DPDK) implementation, sysfs interface
> "/sys/devices/pci/.../resource0_wc" is used to map prefetchable PCI
> resources as WC.
> Platforms which support write-combining maps of PCI resources have
> arch_can_pci_mmap_wc() flag enabled. So that it allows to map resources as WC.
> In this approach mmap calls "pci_mmap_resource_range" kernel function
> with write_combine parameter set.
> "drivers/pci/pci-sysfs.c" kernel file has this implementation.
> 
> If this approach fits to vfio driver, then code change in vfio driver are
> 
>  if (arch_can_pci_mmap_wc() &&
>      (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH))
>             vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>         else
> vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> 
> Please provide your feedback.

Let me see if I've got this straight, UIO (in reality pci-sysfs)
provides a separate interface such that a driver can *choose* to get
either a UC or, at their discretion, a WC mapping to a region, and
you're using that as justification that vfio-pci should arbitrarily
convert all existing users from UC mappings to WC mappings, without
their consent, any time the architecture supports WC mappings.  The fact
that pci-sysfs provides separate interfaces such that drives can choose
their preferred mapping attributes only reaffirms to me that this needs
to be a driver decision.  How would we ever validate that a change like
you're proposing above would not introduce regressions for existing
users?  Thanks,

Alex