linux-kernel - Re: [PATCH] x86/bugs/mmio: Rename mmio_stale_data_clear to cpu_buf_vm

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Message-ID: <Z_hV3pbpM2Y2qq6k@google.com>
Date: Thu, 10 Apr 2025 16:35:58 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] x86/bugs/mmio: Rename mmio_stale_data_clear to cpu_buf_vm_clear

On Thu, Apr 10, 2025, Pawan Gupta wrote:
> On Thu, Apr 10, 2025 at 01:39:28PM -0700, Sean Christopherson wrote:
> > On Thu, Apr 10, 2025, Pawan Gupta wrote:
> > > The static key mmio_stale_data_clear controls the KVM-only mitigation for
> > > MMIO Stale Data vulnerability. Rename it to reflect its purpose.
> > > 
> > > No functional change.
> > > 
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > > ---
> > >  arch/x86/include/asm/nospec-branch.h |  2 +-
> > >  arch/x86/kernel/cpu/bugs.c           | 16 ++++++++++------
> > >  arch/x86/kvm/vmx/vmx.c               |  2 +-
> > >  3 files changed, 12 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > > index 8a5cc8e70439e10aab4eeb5b0f5e116cf635b43d..c0474e2b741737dad129159adf3b5fc056b6097c 100644
> > > --- a/arch/x86/include/asm/nospec-branch.h
> > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > @@ -561,7 +561,7 @@ DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
> > >  
> > >  DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
> > >  
> > > -DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
> > > +DECLARE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
> > 
> > Could we tack on "if_mmio" or something?  E.g. cpu_buf_vm_clear_if_mmio.  FWIW,
> > I don't love that name, so if anyone can come up with something better...
> 
> Keeping it generic has an advantage that it plays nicely with "Attack vector
> controls" series:
> 
>   https://lore.kernel.org/lkml/20250310164023.779191-1-david.kaplan@amd.com/
> 
> The idea being to allow mitigations to be enabled/disabled based on
> user-defined threat model. MDS/TAA mitigations may be able to take
> advantage this KVM-only control.
> 
> > I like the idea of tying the static key back to X86_FEATURE_CLEAR_CPU_BUF, but
> > when looking at just the usage in KVM, "cpu_buf_vm_clear" doesn't provide any
> > hints as to when/why KVM needs to clear buffers.
> 
> Thats fair, can we cover that with a comment like below:
> 
> ---
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c79720aad3df..cddad4a6eb46 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7358,6 +7358,10 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>  	 * mitigation for MDS is done late in VMentry and is still
>  	 * executed in spite of L1D Flush. This is because an extra VERW
>  	 * should not matter much after the big hammer L1D Flush.
> +	 *
> +	 * cpu_buf_vm_clear is used when system is not vulnerable to MDS/TAA,
> +	 * but is affected by MMIO Stale Data that only needs mitigation
> +	 * against a rogue guest.

Would this be accurate?

	 * Clear CPU buffers if the vCPU has access to host MMIO, and the CPU
	 * is NOT vulnerable to MSD/TAA but is affected by MMIO Stale Data, as
	 * indicated by cpu_buf_vm_clear.  If the CPU is vulnerable to MDS/TAA,
	 * CPU buffers will be cleared just before every VM-Entry.

>  	 */
>  	if (static_branch_unlikely(&vmx_l1d_should_flush))
>  		vmx_l1d_flush(vcpu);