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Date: Fri, 3 Nov 2017 14:54:13 +0800 From: Xiao Guangrong <guangrong.xiao@...il.com> To: Haozhong Zhang <haozhong.zhang@...el.com>, kvm@...r.kernel.org, x86@...nel.org Cc: linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com, Xiao Guangrong <xiaoguangrong.eric@...il.com>, Dan Williams <dan.j.williams@...el.com>, ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Olif Chapman <olif.chapman@...cle.com>, Ingo Molnar <mingo@...hat.com> Subject: Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page check On 11/03/2017 01:53 PM, 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 by lookup_memtype() in > addition and only treat UC/UC- pages as MMIO. > > Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com> > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@...el.com> > Reported-by: Kumar, Karthik <karthik.kumar@...el.com> > --- > arch/x86/kvm/mmu.c | 19 ++++++++++++++++++- > 1 file changed, 18 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > index 0b481cc9c725..e9ed0e666a83 100644 > --- a/arch/x86/kvm/mmu.c > +++ b/arch/x86/kvm/mmu.c > @@ -2708,7 +2708,24 @@ 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_pfn_is_uc() works only when PAT is enabled, > + * so check pat_enabled() as well. > + */ > + (!pat_enabled() || > + pat_pfn_is_uc(kvm_pfn_t_to_pfn_t(pfn))); Can it be compiled if !CONFIG_PAT? It would be better if we move pat_enabled out of kvm as well, please refer to pgprot_writecombine() which is implemented in pat.c and in include\asm-generic\pgtable.h: #ifndef pgprot_writecombine #define pgprot_writecombine pgprot_noncached #endif
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