From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755850AbdKCJKT (ORCPT ); Fri, 3 Nov 2017 05:10:19 -0400 Received: from mail-io0-f195.google.com ([209.85.223.195]:46281 "EHLO mail-io0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752309AbdKCJKR (ORCPT ); Fri, 3 Nov 2017 05:10:17 -0400 X-Google-Smtp-Source: ABhQp+TS0YGKhr5t1UC6wirK+i/h924OXykPlS8kCsL8oSSvR99qZ6htT17CK5sMXz4dxTagE4rPXg== Subject: Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page check To: Xiao Guangrong , kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , rkrcmar@redhat.com, Dan Williams , ivan.d.cuevas.escareno@intel.com, karthik.kumar@intel.com, Konrad Rzeszutek Wilk , Olif Chapman , Ingo Molnar References: <20171103055327.21684-1-haozhong.zhang@intel.com> <20171103055327.21684-4-haozhong.zhang@intel.com> <745df599-b65b-bcfe-a20c-05ae6d99a0b5@gmail.com> <20171103085130.itjzxoaorvw34vnm@hz-desktop> From: Xiao Guangrong Message-ID: Date: Fri, 3 Nov 2017 17:10:36 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171103085130.itjzxoaorvw34vnm@hz-desktop> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>> Reported-by: Cuevas Escareno, Ivan D >>> Reported-by: Kumar, Karthik >>> --- >>> 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.