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SubjectRe: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc safe
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>> > +   /*
>> > +    * The VM_ALLOW_ANY_UNCACHED VMA flag is implemented for ARM64,
>> > +    * allowing KVM stage 2 device mapping attributes to use Normal-NC
>> > +    * rather than DEVICE_nGnRE, which allows guest mappings
>> > +    * supporting write-combining attributes (WC). This also
>> > +    * unlocks memory-like operations such as unaligned accesses
>> > +    * This setting suits the fake BARs as they are expected to
>> > +    * demonstrate such properties within the guest.
>> > +    *
>> > +    * ARM does not architecturally guarantee this is safe, and indeed
>> > +    * some MMIO regions like the GICv2 VCPU interface can trigger
>> > +    * uncontained faults if Normal-NC is used. The nvgrace-gpu
>> > +    * however is safe in that the platform guarantees that no
>> > +    * action taken on the MMIO mapping can trigger an uncontained
>> > +    * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags.
>> > +    */
>> > +   vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
>> > +
>> >     return 0;
>> >  }
>> >
>>
>> The commit log sort of covers it, but this comment doesn't seem to
>> cover why we're setting an uncached attribute to the usemem region
>> which we're specifically mapping as coherent... did we end up giving
>> this flag a really poor name if it's being used here to allow unaligned
>> access?  Thanks,
>
> Also, this is setting the vma flag *after* the call to
> remap_pfn_range(), which seems quite sketchy.  Thanks,

I will move it to the block that Jason pointed.

> Alex

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