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Message-ID: <aDjdagbqcesTcnhc@google.com>
Date: Thu, 29 May 2025 15:19:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Borislav Petkov <bp@...en8.de>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 3/5] KVM: VMX: Apply MMIO Stale Data mitigation if KVM
maps MMIO into the guest
On Wed, May 28, 2025, Pawan Gupta wrote:
> On Thu, May 22, 2025 at 06:17:54PM -0700, Sean Christopherson wrote:
> > @@ -7282,7 +7288,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> > if (static_branch_unlikely(&vmx_l1d_should_flush))
> > vmx_l1d_flush(vcpu);
> > else if (static_branch_unlikely(&mmio_stale_data_clear) &&
> > - kvm_arch_has_assigned_device(vcpu->kvm))
> > + (flags & VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO))
> > mds_clear_cpu_buffers();
>
> I think this also paves way for buffer clear for MDS and MMIO to be done at
> a single place. Please let me know if below is feasible:
It's definitely feasible (this thought crossed my mind as well), but because
CLEAR_CPU_BUFFERS emits VERW iff X86_FEATURE_CLEAR_CPU_BUF is enabled, the below
would do nothing for the MMIO case (either that, or I'm missing something).
We could obviously rework CLEAR_CPU_BUFFERS, I'm just not sure that's worth the
effort at this point. I'm definitely not opposed to it though.
> diff --git a/arch/x86/kvm/vmx/run_flags.h b/arch/x86/kvm/vmx/run_flags.h
> index 2f20fb170def..004fe1ca89f0 100644
> --- a/arch/x86/kvm/vmx/run_flags.h
> +++ b/arch/x86/kvm/vmx/run_flags.h
> @@ -2,12 +2,12 @@
> #ifndef __KVM_X86_VMX_RUN_FLAGS_H
> #define __KVM_X86_VMX_RUN_FLAGS_H
>
> -#define VMX_RUN_VMRESUME_SHIFT 0
> -#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT 1
> -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT 2
> +#define VMX_RUN_VMRESUME_SHIFT 0
> +#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT 1
> +#define VMX_RUN_CLEAR_CPU_BUFFERS_SHIFT 2
>
> -#define VMX_RUN_VMRESUME BIT(VMX_RUN_VMRESUME_SHIFT)
> -#define VMX_RUN_SAVE_SPEC_CTRL BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT)
> -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT)
> +#define VMX_RUN_VMRESUME BIT(VMX_RUN_VMRESUME_SHIFT)
> +#define VMX_RUN_SAVE_SPEC_CTRL BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT)
> +#define VMX_RUN_CLEAR_CPU_BUFFERS BIT(VMX_RUN_CLEAR_CPU_BUFFERS_SHIFT)
>
> #endif /* __KVM_X86_VMX_RUN_FLAGS_H */
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index f6986dee6f8c..ab602ce4967e 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -141,6 +141,8 @@ SYM_FUNC_START(__vmx_vcpu_run)
> /* Check if vmlaunch or vmresume is needed */
> bt $VMX_RUN_VMRESUME_SHIFT, %ebx
>
> + test $VMX_RUN_CLEAR_CPU_BUFFERS, %ebx
> +
> /* Load guest registers. Don't clobber flags. */
> mov VCPU_RCX(%_ASM_AX), %_ASM_CX
> mov VCPU_RDX(%_ASM_AX), %_ASM_DX
> @@ -161,8 +163,11 @@ SYM_FUNC_START(__vmx_vcpu_run)
> /* Load guest RAX. This kills the @regs pointer! */
> mov VCPU_RAX(%_ASM_AX), %_ASM_AX
>
> + /* Check EFLAGS.ZF from the VMX_RUN_CLEAR_CPU_BUFFERS bit test above */
> + jz .Lskip_clear_cpu_buffers
> /* Clobbers EFLAGS.ZF */
> CLEAR_CPU_BUFFERS
> +.Lskip_clear_cpu_buffers:
>
> /* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
> jnc .Lvmlaunch
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1e4790c8993a..1415aeea35f7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -958,9 +958,10 @@ unsigned int __vmx_vcpu_run_flags(struct vcpu_vmx *vmx)
> if (!msr_write_intercepted(vmx, MSR_IA32_SPEC_CTRL))
> flags |= VMX_RUN_SAVE_SPEC_CTRL;
>
> - if (static_branch_unlikely(&mmio_stale_data_clear) &&
> - kvm_vcpu_can_access_host_mmio(&vmx->vcpu))
> - flags |= VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO;
> + if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF) ||
> + (static_branch_unlikely(&mmio_stale_data_clear) &&
> + kvm_vcpu_can_access_host_mmio(&vmx->vcpu)))
> + flags |= VMX_RUN_CLEAR_CPU_BUFFERS;
>
> return flags;
> }
> @@ -7296,9 +7297,6 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> */
> if (static_branch_unlikely(&vmx_l1d_should_flush))
> vmx_l1d_flush(vcpu);
> - else if (static_branch_unlikely(&mmio_stale_data_clear) &&
> - (flags & VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO))
> - mds_clear_cpu_buffers();
>
> vmx_disable_fb_clear(vmx);
>
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