Re: [PATCH v12 08/20] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA

From: David Woodhouse <dwmw2@infradead.org>
To: Sean Christopherson <seanjc@google.com>, 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>,
	 Shuah Khan <shuah@kernel.org>,
	kvm@vger.kernel.org, linux-doc@vger.kernel.org,
	 linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v12 08/20] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA
Date: Tue, 06 Feb 2024 20:13:09 -0800	[thread overview]
Message-ID: <7c31e31d0f0ad3f40619f8e0ecf67f1e3d3eef5f.camel@infradead.org> (raw)
In-Reply-To: <ZcMBDP6H5PRo5C2d@google.com>

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On Tue, 2024-02-06 at 20:03 -0800, Sean Christopherson wrote:
> +s390 folks (question on kvm_is_error_gpa() for ya)
> 
> On Mon, Jan 15, 2024, Paul Durrant wrote:
> > @@ -1398,7 +1414,9 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc);
> >   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);
> > +
> > +       if (gpc->gpa != KVM_XEN_INVALID_GPA)
> 
> KVM_XEN_INVALID_GPA absolutely doesn't belong in common code.  Not to mention
> that it will break when Paolo (rightly) moves it to an x86 header.
> 
> https://lore.kernel.org/all/20240131233056.10845-3-pbonzini@redhat.com

We can use plain INVALID_GPA for that, I think. ISTR the reason we have
a separate KVM_XEN_INVALID_GPA is because that's a userspace API.

...

> But!  kvm_is_error_gpa() already exists, and it very, very sneakily
> does a memslot lookup and checks for a valid HVA.

Hm, that doesn't sound as fast as simple comparison. We also can't do
it from kvm_gpc_check(), can we?

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