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

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Message-ID: <Zczj2vDCasKcBcjn@google.com>
Date: Wed, 14 Feb 2024 08:01:30 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Paul Durrant <paul@....org>, 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, Feb 06, 2024, David Woodhouse wrote:
> 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?

You snipped the part where I suggested renaming the existing kvm_is_error_gpa().

I am suggesting we do the below (and obviously rename the s390 usage, too), and
then the gpc code can use use kvm_is_error_gpa().

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index bbfefd7e612f..e1df988e4d57 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -148,6 +148,11 @@ static inline bool kvm_is_error_hva(unsigned long addr)
 
 #endif
 
+static inline bool kvm_is_error_gpa(gpa_t gpa)
+{
+       return gpa == INVALID_GPA;
+}
+
 #define KVM_ERR_PTR_BAD_PAGE   (ERR_PTR(-ENOENT))
 
 static inline bool is_error_page(struct page *page)
@@ -1787,7 +1792,7 @@ static inline hpa_t pfn_to_hpa(kvm_pfn_t pfn)
        return (hpa_t)pfn << PAGE_SHIFT;
 }
 
-static inline bool kvm_is_error_gpa(struct kvm *kvm, gpa_t gpa)
+static inline bool kvm_gpa_is_in_memslot(struct kvm *kvm, gpa_t gpa)
 {
        unsigned long hva = gfn_to_hva(kvm, gpa_to_gfn(gpa));