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Date: Fri, 3 Nov 2017 17:10:36 +0800 From: Xiao Guangrong <xiaoguangrong.eric@...il.com> To: Xiao Guangrong <guangrong.xiao@...il.com>, kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.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 04:51 PM, Haozhong Zhang wrote: > On 11/03/17 14:54 +0800, Xiao Guangrong wrote: >> >> >> 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? > > Yes. > > What I check via pat_enabled() is not only whether PAT support is > compiled, but also whether PAT is enabled at runtime. The issue is about pat_pfn_is_uc() which is implemented only if CONFIG_PAT is enabled, but you used it here unconditionally. I am not sure if gcc is smart enough to omit pat_pfn_is_uc() completely under this case. If you really have done the test to compile kernel and KVM module with CONFIG_PAT disabled, it is fine.
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