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

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Date: Tue, 06 Feb 2024 20:13:09 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>, Paul Durrant <paul@....org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,  Shuah Khan
 <shuah@...nel.org>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, Christian
 Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank
 <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, David
 Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v12 08/20] KVM: pfncache: allow a cache to be activated
 with a fixed (userspace) HVA

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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