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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id h12sm3180823edb.16.2021.01.28.09.25.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 28 Jan 2021 09:25:05 -0800 (PST) Subject: Re: [PATCH v2 05/14] KVM: x86: Override reported SME/SEV feature flags with host mask To: Sean Christopherson Cc: Dave Hansen , Andy Lutomirski , Peter Zijlstra , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Borislav Petkov , Tom Lendacky , Brijesh Singh References: <20210114003708.3798992-1-seanjc@google.com> <20210114003708.3798992-6-seanjc@google.com> <74642db3-14dc-4e13-3130-dc8abe1a2b6e@redhat.com> From: Paolo Bonzini Message-ID: <8fa5e165-6f93-572f-81d5-07cdeb23b590@redhat.com> Date: Thu, 28 Jan 2021 18:25:03 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 28/01/21 18:09, Sean Christopherson wrote: > On Thu, Jan 28, 2021, Paolo Bonzini wrote: >> On 14/01/21 01:36, Sean Christopherson wrote: >>> Add a reverse-CPUID entry for the memory encryption word, 0x8000001F.EAX, >>> and use it to override the supported CPUID flags reported to userspace. >>> Masking the reported CPUID flags avoids over-reporting KVM support, e.g. >>> without the mask a SEV-SNP capable CPU may incorrectly advertise SNP >>> support to userspace. >>> >>> Cc: Brijesh Singh >>> Cc: Tom Lendacky >>> Signed-off-by: Sean Christopherson >>> --- >>> arch/x86/kvm/cpuid.c | 2 ++ >>> arch/x86/kvm/cpuid.h | 1 + >>> 2 files changed, 3 insertions(+) >>> >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >>> index 13036cf0b912..b7618cdd06b5 100644 >>> --- a/arch/x86/kvm/cpuid.c >>> +++ b/arch/x86/kvm/cpuid.c >>> @@ -855,6 +855,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function) >>> case 0x8000001F: >>> if (!boot_cpu_has(X86_FEATURE_SEV)) >>> entry->eax = entry->ebx = entry->ecx = entry->edx = 0; >>> + else >>> + cpuid_entry_override(entry, CPUID_8000_001F_EAX); >>> break; >>> /*Add support for Centaur's CPUID instruction*/ >>> case 0xC0000000: >>> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h >>> index dc921d76e42e..8b6fc9bde248 100644 >>> --- a/arch/x86/kvm/cpuid.h >>> +++ b/arch/x86/kvm/cpuid.h >>> @@ -63,6 +63,7 @@ static const struct cpuid_reg reverse_cpuid[] = { >>> [CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX}, >>> [CPUID_7_EDX] = { 7, 0, CPUID_EDX}, >>> [CPUID_7_1_EAX] = { 7, 1, CPUID_EAX}, >>> + [CPUID_8000_001F_EAX] = {0x8000001f, 1, CPUID_EAX}, >>> }; >>> /* >>> >> >> I don't understand, wouldn't this also need a kvm_cpu_cap_mask call >> somewhere else? As it is, it doesn't do anything. > > Ugh, yes, apparently I thought the kernel would magically clear bits it doesn't > care about. > > Looking at this again, I think the kvm_cpu_cap_mask() invocation should always > mask off X86_FEATURE_SME. SME cannot be virtualized, and AFAIK it's not > emulated by KVM. This would fix an oddity where SME would be advertised if SEV > is also supported. > > Boris has queue the kernel change to tip/x86/cpu, I'll spin v4 against that. You can send it after the 5.12 merge window. Paolo