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AJvYcCVQ9KkEL6lLw4NysTeYfd36zU513OP0R/dINnJmcFxoUEg00frg+zdhFizZeDFw2ZQBnlQ=@vger.kernel.org X-Gm-Message-State: AOJu0Yx1vJYcCgrNyuSVWhnJKx5IcSgNiqvw91wU9qEr5y+479Qxa9wZ rbTlBaH5ufQnmR9CBxVKluriGvU9VrPwJzgbFrj0QZ/eduNgJ4y7B9iTRJKDTMtR2hruQTyovMy GS9Ax4Q== X-Google-Smtp-Source: AGHT+IFHpp7bNY375rv69dfJVKRP14JO7bDZ6Qzp4pJXKIQ9Tx1TN/S1z7ykvM7IVkTTEWv8EepdbXFLQ7U= X-Received: from pgbcw3.prod.google.com ([2002:a05:6a02:4283:b0:bac:a20:5eec]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:3d09:b0:346:331:97e4 with SMTP id adf61e73a8af0-3590bd0b3e8mr5777964637.56.1762979438181; Wed, 12 Nov 2025 12:30:38 -0800 (PST) Date: Wed, 12 Nov 2025 12:30:36 -0800 In-Reply-To: <20251112183836.GBaRTULLaMWA5hkfT9@fat_crate.local> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251031003040.3491385-1-seanjc@google.com> <20251031003040.3491385-5-seanjc@google.com> <20251112164144.GAaRS4yKgF0gQrLSnR@fat_crate.local> <20251112183836.GBaRTULLaMWA5hkfT9@fat_crate.local> Message-ID: Subject: Re: [PATCH v4 4/8] KVM: VMX: Handle MMIO Stale Data in VM-Enter assembly via ALTERNATIVES_2 From: Sean Christopherson To: Borislav Petkov Cc: Paolo Bonzini , Thomas Gleixner , Peter Zijlstra , Josh Poimboeuf , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Pawan Gupta , Brendan Jackman Content-Type: text/plain; charset="us-ascii" On Wed, Nov 12, 2025, Borislav Petkov wrote: > On Wed, Nov 12, 2025 at 09:15:00AM -0800, Sean Christopherson wrote: > > On Wed, Nov 12, 2025, Borislav Petkov wrote: > > > So this VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO bit gets set here: > > > > > > if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_MMIO) && > > > kvm_vcpu_can_access_host_mmio(&vmx->vcpu)) > > > flags |= VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO; > > > > > > So how static and/or dynamic is this? > > > > kvm_vcpu_can_access_host_mmio() is very dynamic. It can be different between > > vCPUs in a VM, and can even change on back-to-back runs of the same vCPU. > > Hmm, strange. Because looking at those things there: > > root->has_mapped_host_mmio and vcpu->kvm->arch.has_mapped_host_mmio > > they both read like something that a guest would set up once and that's it. > But what do I know... They're set based on what memory is mapped into the KVM-controlled page tables, e.g. into the EPT/NPT tables, that will be used by the vCPU for that VM-Enter. root->has_mapped_host_mmio is per page table. vcpu->kvm->arch.has_mapped_host_mmio exists because of nastiness related to shadow paging; for all intents and purposes, I would just mentally ignore that one. > > > IOW, can you stick this into a simple variable which is unconditionally > > > updated and you can use it in X86_FEATURE_CLEAR_CPU_BUF_MMIO case and > > > otherwise it simply remains unused? > > > > Can you elaborate? I don't think I follow what you're suggesting. > > So I was thinking if you could set a per-guest variable in > C - vmx_per_guest_clear_per_mmio or so and then test it in asm: > > testb $1,vmx_per_guest_clear_per_mmio(%rip) > jz .Lskip_clear_cpu_buffers; > CLEAR_CPU_BUFFERS_SEQ; > > .Lskip_clear_cpu_buffers: > > gcc -O3 suggests also > > cmpb $0x0,vmx_per_guest_clear_per_mmio(%rip) > > which is the same insn size... > > The idea is to get rid of this first asm stashing things and it'll be a bit > more robust, I'd say. VMX "needs" to abuse RFLAGS no matter what, because RFLAGS is the only register that's available at the time of VMLAUNCH/VMRESUME. On Intel, only RSP and RFLAGS are context switched via the VMCS, all other GPRs need to be context switch by software. Which is why I didn't balk at Pawan's idea to use RFLAGS.ZF to track whether or not a VERW for MMIO is needed. Hmm, actually, @flags is already on the stack because it's needed at VM-Exit. Using EBX was a holdover from the conversion from inline asm to "proper" asm, e.g. from commit 77df549559db ("KVM: VMX: Pass @launched to the vCPU-run asm via standard ABI regs"). Oooh, and if we stop using bt+RFLAGS.CF, then we drop the annoying SHIFT definitions in arch/x86/kvm/vmx/run_flags.h. Very lightly tested at this point, but I think this can all be simplified to /* * Note, ALTERNATIVE_2 works in reverse order. If CLEAR_CPU_BUF_VM is * enabled, do VERW unconditionally. If CPU_BUF_VM_MMIO is enabled, * check @flags to see if the vCPU has access to host MMIO, and do VERW * if so. Else, do nothing (no mitigations needed/enabled). */ ALTERNATIVE_2 "", \ __stringify(testl $VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO, WORD_SIZE(%_ASM_SP); \ jz .Lskip_clear_cpu_buffers; \ VERW; \ .Lskip_clear_cpu_buffers:), \ X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO, \ __stringify(VERW), X86_FEATURE_CLEAR_CPU_BUF_VM /* Check if vmlaunch or vmresume is needed */ testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP) jz .Lvmlaunch > And you don't rely on registers... > > and when I say that, I now realize this is 32-bit too and you don't want to > touch regs - that's why you're stashing it - and there's no rip-relative on > 32-bit... > > I dunno - it might get hairy but I would still opt for a different solution > instead of this fragile stashing in ZF. You could do a function which pushes > and pops a scratch register where you put the value, i.e., you could do > > push %reg > mov var, %reg > test or cmp ... > ... > jz skip... > skip: > pop %reg > > It is still all together in one place instead of spreading it around like > that. FWIW, all GPRs except RSP are off limits. But as above, getting at @flags via RSP is trivial.