RE: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

[Vikram Sethi]

Hi Marc, 

> From: Marc Zyngier <maz@xxxxxxxxxx>
> Hi Vikram,
> 
 
> The problem I see is that we have VM and userspace being written in terms
> of Write-Combine, which is:
> 
> - loosely defined even on x86
> 
> - subject to interpretations in the way it maps to PCI
> 
> - has no direct equivalent in the ARMv8 collection of memory
>   attributes (and Normal_NC comes with speculation capabilities which
>   strikes me as extremely undesirable on arbitrary devices)

If speculation with Normal NC to prefetchable BARs in devices was a problem, 
those devices would already be broken in baremetal with ioremap_wc on arm64, 
and we would need quirks there to not do Normal NC for them but Device GRE, 
and if such a quirk was needed on baremetal, it could be picked up by vfio/KVM
as well. But we haven't seen any broken devices doing wc on baremetal on ARM64, have we?
I know we have tested NICs write combining on arm64 in baremetal, as well as GPU
and NVMe CMB without issues.

Further, I don't see why speculation to non cacheble would be an issue if prefetch 
without side effects is allowed by the device, which is what a prefetchable BAR is. 
If it is an issue for a device I would consider that a bug already needing a quirk in
Baremetal/host kernel already. 
From PCI spec " A prefetchable address range may have write side effects, 
but it may not have read side effects."
> 
> How do we translate this into something consistent? I'd like to see an actual
> description of what we *really* expect from WC on prefetchable PCI regions,
> turn that into a documented definition agreed across architectures, and then
> we can look at implementing it with one memory type or another on arm64.
> 
> Because once we expose that memory type at S2 for KVM guests, it
> becomes ABI and there is no turning back. So I want to get it right once and
> for all.
> 
I agree that we need a precise definition for the Linux ioremap_wc API wrt what
drivers (kernel and userspace) can expect and whether memset/memcpy is expected
to work or not and whether aligned accesses are a requirement. 
To the extent ABI is set, I would think that the ABI is also already set in the host kernel 
for arm64 WC = Normal NC, so why should that not also be the ABI for same driver in VMs.

> Thanks,
> 
>         M.
> 
> --
> Without deviation from the norm, progress is not possible.