>> > >> > + /* >> > + * 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, > > Yeah, I sugged to fold that hunk into this: > > if (index == RESMEM_REGION_INDEX) > vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot); > > So it makes more sense. VM_ALLOW_ANY_UNCACHED shouldn't be used on the > cachable mapping. The comment should be more specific to this driver > and not so generic: > > /* > * nvgrace has no issue with uncontained failures on NORMAL_NC > * access. Tell KVM to open up guest usage of NORMAL_NC for this mapping. > */ Sure, I will restrict this to resmem. Also will update the comment accordingly. > > Jason
