On 08/11/2017 08:56, Haozhong Zhang wrote:
> Some reserved pages, such as those from NVDIMM DAX devices, are not
> for MMIO, and can be mapped with cached memory type for better
> performance. However, the above check misconceives those pages as
> MMIO. Because KVM maps MMIO pages with UC memory type, the
> performance of guest accesses to those pages would be harmed.
> Therefore, we check the host memory type in addition and only treat
> UC/UC- pages as MMIO.
>
> Signed-off-by: Haozhong Zhang <[email protected]>
> Reported-by: Cuevas Escareno, Ivan D <[email protected]>
> Reported-by: Kumar, Karthik <[email protected]>
WC should be allowed as well, because the combination of EPT_PAT=UC and
gPAT=WC gives WC effective memory type.
Maybe it's better after all if the lookup_memtype call remains in
kvm_is_mmio_pfn, like
if (!pfn_valid(pfn))
return true;
if (is_zero_pfn(pfn) || !PageReserved(pfn_to_page(pfn))
return false;
/* ... long comment ... */
if (!pat_enabled()
return true;
else {
cm = lookup_memtype(PFN_PHYS(pfn));
return cm == ...
}
or something like that.
Thanks,
Paolo
> ---
> arch/x86/kvm/mmu.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0b481cc9c725..7715476bc5c9 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2708,7 +2708,20 @@ 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));
> + return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
> + /*
> + * Some reserved pages, such as those from
> + * NVDIMM DAX devices, are not for MMIO, and
> + * can be mapped with cached memory type for
> + * better performance. However, the above
> + * check misconceives those pages as MMIO.
> + * Because KVM maps MMIO pages with UC memory
> + * type, the performance of guest accesses to
> + * those pages would be harmed. Therefore, we
> + * check the host memory type in addition and
> + * only treat UC/UC- pages as MMIO.
> + */
> + (!pat_enabled() || pat_pfn_is_uc_or_uc_minus(pfn));
>
> return true;
> }
>
