Re: [PATCH 2/4] KVM: SEV: Drop page refcount early during RMP fault handling

[Yan Zhao]

On Tue, Aug 18, 2026 at 08:12:36AM +0000, Ackerley Tng wrote:
> When handling an RMP fault, KVM retrieves the PFN for a private GPA from
> guest_memfd. The page reference taken during PFN lookup is not needed for
> the remainder of the handler, as checking the RMP entry, splitting the
> 2MB page via PSMASH, and zapping shadow page tables only operate on PFNs.
> 
> Drop the page reference immediately after retrieving the PFN instead of
> holding it across the entire handler.
> 
> A later patch will follow up with completely not returning refcounted pages
> from kvm_gmem_get_pfn().
> 
> No functional change intended.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/sev.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b2738362a928b..19c31fa944e34 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -5036,12 +5036,13 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
>  				    gpa);
>  		return;
>  	}
> +	kvm_release_page_unused(page);
>  
>  	ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
>  	if (ret || !assigned) {
>  		pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
>  				    gpa, pfn, ret);
> -		goto out_no_trace;
> +		return;
>  	}
Hi Ackerley,                                                                     
 
You missed the parts in [1] which save mmu_seq before invoking kvm_gmem_get_pfn(),
+       mmu_seq = kvm->mmu_invalidate_seq;
+       smp_rmb();

and check mmu_invalidate_retry_gfn() before snp_rmptable_psmash(pfn):

+       scoped_guard(read_lock)(&kvm->mmu_lock) {                                
+               if (mmu_invalidate_retry_gfn(kvm, mmu_seq, gfn))                 
                        goto out;                                                

[1] https://lore.kernel.org/all/an5RJYTwlYeym--O@xxxxxxxxxx/    
>  	/*
> @@ -5088,8 +5089,6 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
>  	kvm_zap_gfn_range(kvm, gfn, gfn + PTRS_PER_PMD);
>  out:
>  	trace_kvm_rmp_fault(vcpu, gpa, pfn, error_code, rmp_level, ret);
> -out_no_trace:
> -	kvm_release_page_unused(page);
>  }
>  
>  static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)
> 
> -- 
> 2.55.0.699.gb54405d56f-goog
>