Messages in this thread Patch in this message | | | Subject | Re: [PATCH v5 2/2] KVM: MMU: consider host cache mode in MMIO page check | From | Paolo Bonzini <> | Date | Fri, 15 Dec 2017 10:14:24 +0100 |
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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 <haozhong.zhang@intel.com> > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com> > Reported-by: Kumar, Karthik <karthik.kumar@intel.com>
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; > } >
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