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Thu, 04 Jul 2024 18:21:34 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGicR0okBEqG0PULE/EG8W94vYsVFsQcCKsh9Xj7NWrddJl1sZNVYVh4JC0ge2nMXeETH5RYw== X-Received: by 2002:a05:6214:daf:b0:6b5:dff6:cac0 with SMTP id 6a1803df08f44-6b5ecf97793mr38138096d6.21.1720142494406; Thu, 04 Jul 2024 18:21:34 -0700 (PDT) Received: from starship ([2607:fea8:fc01:7b7f:6adb:55ff:feaa:b156]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6b59e5f39c1sm68802036d6.97.2024.07.04.18.21.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Jul 2024 18:21:34 -0700 (PDT) Message-ID: <2a4052ba67970ce41e79deb0a0931bb54e2c2a86.camel@redhat.com> Subject: Re: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features that ignore host kernel support From: Maxim Levitsky To: Sean Christopherson , Paolo Bonzini , Vitaly Kuznetsov Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Hou Wenlong , Kechen Lu , Oliver Upton , Binbin Wu , Yang Weijiang , Robert Hoo Date: Thu, 04 Jul 2024 21:21:33 -0400 In-Reply-To: <20240517173926.965351-20-seanjc@google.com> References: <20240517173926.965351-1-seanjc@google.com> <20240517173926.965351-20-seanjc@google.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote: > Add a macro for use in kvm_set_cpu_caps() to automagically initialize > features that KVM wants to support based solely on the CPU's capabilities, > e.g. KVM advertises LA57 support if it's available in hardware, even if > the host kernel isn't utilizing 57-bit virtual addresses. > > Take advantage of the fact that kvm_cpu_cap_mask() adjusts kvm_cpu_caps > based on raw CPUID, i.e. will clear features bits that aren't supported in > hardware, and simply force-set the capability before applying the mask. > > Abusing kvm_cpu_cap_set() is a borderline evil shenanigan, but doing so > avoid extra CPUID lookups, and a future commit will harden the entire > family of *F() macros to assert (at compile time) that every feature being > allowed is part of the capability word being processed, i.e. using a macro > will bring more advantages in the future. Could you explain what do you mean by "extra CPUID lookups"? > > Avoiding CPUID also fixes a largely benign bug where KVM could incorrectly > report LA57 support on Intel CPUs whose max supported CPUID is less than 7, > i.e. if the max supported leaf (<7) happened to have bit 16 set. In > practice, barring a funky virtual machine setup, the bug is benign as all > known CPUs that support VMX also support leaf 7. > > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/cpuid.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 77625a5477b1..a802c09b50ab 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -70,6 +70,18 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted) > (boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0); \ > }) > > +/* > + * Raw Feature - For features that KVM supports based purely on raw host CPUID, > + * i.e. that KVM virtualizes even if the host kernel doesn't use the feature. > + * Simply force set the feature in KVM's capabilities, raw CPUID support will > + * be factored in by kvm_cpu_cap_mask(). > + */ > +#define RAW_F(name) \ > +({ \ > + kvm_cpu_cap_set(X86_FEATURE_##name); \ > + F(name); \ > +}) > + > /* > * Magic value used by KVM when querying userspace-provided CPUID entries and > * doesn't care about the CPIUD index because the index of the function in > @@ -682,15 +694,12 @@ void kvm_set_cpu_caps(void) > F(AVX512VL)); > > kvm_cpu_cap_mask(CPUID_7_ECX, > - F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) | > + F(AVX512VBMI) | RAW_F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) | > F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) | > F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) | > F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ | > F(SGX_LC) | F(BUS_LOCK_DETECT) > ); > - /* Set LA57 based on hardware capability. */ > - if (cpuid_ecx(7) & F(LA57)) > - kvm_cpu_cap_set(X86_FEATURE_LA57); > > /* > * PKU not yet implemented for shadow paging and requires OSPKE Putting a function call into a macro which evaluates into a bitmask is somewhat misleading, but let it be... IMHO in long term, it might be better to rip the whole huge 'or'ed mess, and replace it with a list of statements, along with comments for all unusual cases. Reviewed-by: Maxim Levitsky Best regards, Maxim Levitsky