linux-kernel - Re: [PATCH] kvm/x86: simplify kvm_mmu_do_page

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Message-ID: <ZxfaU9cCS6556AKg@google.com>
Date: Tue, 22 Oct 2024 10:01:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Juergen Gross <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] kvm/x86: simplify kvm_mmu_do_page_fault() a little bit

On Tue, Oct 22, 2024, Juergen Gross wrote:
> Testing whether to call kvm_tdp_page_fault() or
> vcpu->arch.mmu->page_fault() doesn't make sense, as kvm_tdp_page_fault()
> is selected only if vcpu->arch.mmu->page_fault == kvm_tdp_page_fault.

It does when retpolines are enabled and significantly inflate the cost of the
indirect call.  This is a hot path in various scenarios, but KVM can't use
static_call() to avoid the retpoline due to mmu->page_fault being a property of
the current vCPU.  Only kvm_tdp_page_fault() is special cased because all other
mmu->page_fault targets are slow-ish and/or we don't care terribly about their
performance.

> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/kvm/mmu/mmu_internal.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index c98827840e07..6eae54aa1160 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -322,10 +322,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  		fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn);
>  	}
>  
> -	if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && fault.is_tdp)
> -		r = kvm_tdp_page_fault(vcpu, &fault);
> -	else
> -		r = vcpu->arch.mmu->page_fault(vcpu, &fault);
> +	r = vcpu->arch.mmu->page_fault(vcpu, &fault);
>  
>  	/*
>  	 * Not sure what's happening, but punt to userspace and hope that
> -- 
> 2.43.0
>