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Date: Fri, 9 Feb 2024 07:58:59 -0800
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Subject: Re: [PATCH v12 04/20] KVM: pfncache: add a mark-dirty helper
From: Sean Christopherson <seanjc@google.com>
To: Paul Durrant <paul@xen.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, David Woodhouse <dwmw2@infradead.org>, Shuah Khan <shuah@kernel.org>, 
	kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
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On Mon, Jan 15, 2024, Paul Durrant wrote:
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7e7fd25b09b3..f3bb9e0a81fe 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1399,6 +1399,17 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len);
>   */
>  void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc);
>  
> +/**
> + * kvm_gpc_mark_dirty - mark a cached page as dirty.
> + *
> + * @gpc:	   struct gfn_to_pfn_cache object.
> + */
> +static inline void kvm_gpc_mark_dirty(struct gfn_to_pfn_cache *gpc)
> +{
> +	lockdep_assert_held(&gpc->lock);
> +	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);

Can you opportunistically have this pre-check gpc->memslot?  __kvm_gpc_refresh()
should nullify gpc->memslot when using an hva.  That way, you don't need to
explicitly check for the "invalid gfn" case here (or you could, but WARN_ON_ONCE()
if the memslot is non-NULL and the gfn is invalid?).