* Haozhong Zhang <haozhong.zhang@xxxxxxxxx> wrote:
> By default, KVM treats a reserved page as for MMIO purpose, and maps
> it to guest with UC memory type. However, some reserved pages are not
> for MMIO, such as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> them with UC memory type will harm the performance. In order to
> exclude those cases, we check the host cache mode in addition and only
> treat UC/UC- pages as MMIO.
>
> Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@xxxxxxxxx>
> Reported-by: Kumar, Karthik <karthik.kumar@xxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 32 +++++++++++++++++++++++++++++---
> 1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0b481cc9c725..d4c821a6df3d 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2707,10 +2707,36 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
>
> static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
> {
> - if (pfn_valid(pfn))
> - return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
> + bool is_mmio = true;
>
> - return true;
> + if (pfn_valid(pfn)) {
> + is_mmio = !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
> +
> + /*
> + * By default, KVM treats a reserved page as for MMIO
> + * purpose, and maps it to guest with UC memory type.
> + * However, some reserved pages are not for MMIO, such
> + * as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> + * them with UC memory type will harm the performance.
> + * In order to exclude those cases, we check the host
> + * cache mode in addition and only treat UC/UC- pages
> + * as MMIO.
> + *
> + * track_pfn_insert() works only when PAT is enabled,
> + * so add pat_enabled() here.
> + */
> + if (is_mmio && pat_enabled()) {
> + pgprot_t prot;
> + enum page_cache_mode cm;
> +
> + track_pfn_insert(NULL, &prot, kvm_pfn_to_pfn(pfn));
> + cm = pgprot2cachemode(prot);
> + is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
> + cm == _PAGE_CACHE_MODE_UC_MINUS);
> + }
> + }
> +
> + return is_mmio;
> }
>
> static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
s/harm the performance
/harm performance>
But I suspect the rest of the comment should be rewritten too to be more fluid.
Beyond that - I think instead of exposing these low level details a properly named
helper function should be put into pat.c instead - and KVM can use that.
Thanks,
Ingo
