linux-kernel - Re: [PATCH v2 2/8] KVM: SVM: replace regs argument of __svm_vcpu_run with vcpu

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Date:   Wed, 9 Nov 2022 11:35:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        nathan@...nel.org, thomas.lendacky@....com,
        andrew.cooper3@...rix.com, peterz@...radead.org,
        jmattson@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/8] KVM: SVM: replace regs argument of __svm_vcpu_run
 with vcpu_svm

On 11/8/22 21:54, Sean Christopherson wrote:
> On Tue, Nov 08, 2022, Paolo Bonzini wrote:
>> Since registers are reachable through vcpu_svm, and we will
>> need to access more fields of that struct, pass it instead
>> of the regs[] array.
>>
>> No functional change intended.
>>
>> Cc: stable@...r.kernel.org
>> Fixes: f14eec0a3203 ("KVM: SVM: move more vmentry code to assembly")
> 
> I don't think a Fixes: tag for a pure nop patch is fair to the original commit.

That's for the sake of correct tracking in stable@, still it's not the 
right commit to point at:

- f14eec0a3203 did not move the RSB stuffing before the GSBASE restore, 
the code before was:

	vmexit_fill_RSB();
  #ifdef CONFIG_X86_64
  	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
  #else

- anyway it wasn't really buggy at the time: it's only in -next that 
FILL_RETURN_BUFFER uses percpu data, because alternatives take care of 
the X86_FEATURE_* checks

The real reason to do all this is to access the percpu host spec_ctrl, 
which in turn is needed for retbleed.  I'll point the Fixes tag to 
a149180fbcf3 ("x86: Add magic AMD return-thunk") instead, again just for 
the sake of tracking releases that need the change to full fix retbleed.

Paolo