linux-kernel - Re: [PATCH 2/5] KVM: e500: use shadow TLB entry as witness for writability

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Message-ID: <Z4Ac8Xw4ELtHGAHo@google.com>
Date: Thu, 9 Jan 2025 11:01:05 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, oliver.upton@...ux.dev, 
	Will Deacon <will@...nel.org>, Anup Patel <apatel@...tanamicro.com>, 
	Andrew Jones <ajones@...tanamicro.com>, linuxppc-dev@...ts.ozlabs.org, 
	regressions@...ts.linux.dev
Subject: Re: [PATCH 2/5] KVM: e500: use shadow TLB entry as witness for writability

On Thu, Jan 09, 2025, Paolo Bonzini wrote:
> kvmppc_e500_ref_setup is returning whether the guest TLB entry is writable,
> which is than passed to kvm_release_faultin_page.  This makes little sense

s/than/then

> for two reasons: first, because the function sets up the private data for
> the page and the return value feels like it has been bolted on the side;
> second, because what really matters is whether the _shadow_ TLB entry is
> writable.  If it is not writable, the page can be released as non-dirty.
> Shift from using tlbe_is_writable(gtlbe) to doing the same check on
> the shadow TLB entry.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/powerpc/kvm/e500_mmu_host.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
> index 732335444d68..06e23c625be0 100644
> --- a/arch/powerpc/kvm/e500_mmu_host.c
> +++ b/arch/powerpc/kvm/e500_mmu_host.c
> @@ -242,7 +242,7 @@ static inline int tlbe_is_writable(struct kvm_book3e_206_tlb_entry *tlbe)
>  	return tlbe->mas7_3 & (MAS3_SW|MAS3_UW);
>  }
>  
> -static inline bool kvmppc_e500_ref_setup(struct tlbe_ref *ref,
> +static inline void kvmppc_e500_ref_setup(struct tlbe_ref *ref,
>  					 struct kvm_book3e_206_tlb_entry *gtlbe,
>  					 kvm_pfn_t pfn, unsigned int wimg)
>  {
> @@ -251,8 +251,6 @@ static inline bool kvmppc_e500_ref_setup(struct tlbe_ref *ref,
>  
>  	/* Use guest supplied MAS2_G and MAS2_E */
>  	ref->flags |= (gtlbe->mas2 & MAS2_ATTRIB_MASK) | wimg;
> -
> -	return tlbe_is_writable(gtlbe);
>  }
>  
>  static inline void kvmppc_e500_ref_release(struct tlbe_ref *ref)
> @@ -493,10 +491,11 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>  			goto out;
>  		}
>  	}
> -	writable = kvmppc_e500_ref_setup(ref, gtlbe, pfn, wimg);
>  
> +	kvmppc_e500_ref_setup(ref, gtlbe, pfn, wimg);
>  	kvmppc_e500_setup_stlbe(&vcpu_e500->vcpu, gtlbe, tsize,
>  				ref, gvaddr, stlbe);
> +	writable = tlbe_is_writable(stlbe);
>  
>  	/* Clear i-cache for new pages */
>  	kvmppc_mmu_flush_icache(pfn);
> -- 
> 2.47.1
>