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

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On 07/02/2024 04:03, 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@xxxxxxxxxx

+		mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpc->gpa >> PAGE_SHIFT);
  }
void kvm_sigset_activate(struct kvm_vcpu *vcpu);
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 97eec8ee3449..ae822bff812f 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -48,7 +48,10 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
  	if (!gpc->active)
  		return false;
- if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
+	if (gpc->gpa != KVM_XEN_INVALID_GPA && gpc->generation != slots->generation)

This needs a comment.  I know what it's doing, but it wasn't obvious at first
glance, and it definitely won't be intuitive for readers that aren't intimately
familiar with memslots.

+		return false;
+
+	if (kvm_is_error_hva(gpc->uhva))
  		return false;
if (offset_in_page(gpc->uhva) + len > PAGE_SIZE)
@@ -209,11 +212,13 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
  	return -EFAULT;
  }
-static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
+static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
  			     unsigned long len)
  {
  	struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
-	unsigned long page_offset = offset_in_page(gpa);
+	unsigned long page_offset = (gpa != KVM_XEN_INVALID_GPA) ?
+		offset_in_page(gpa) :
+		offset_in_page(uhva);

This formatting is funky.  I also think it would be worth adding a helper to pair
with kvm_is_error_hva().

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

s390 people, any objection to renaming kvm_is_error_gpa() to something like
kvm_gpa_has_memslot() or kvm_gpa_is_in_memslot()?  s390 is the only code that
uses the existing helper.

That would both to free up the name to pair with kvm_is_error_hva(), and would
make it obvious what the helper does; I was quite surprised that "error" means
"is covered by a valid memslot".


Seemingly no response to this; I'll define a local helper rather than re-working the open-coded tests to check against INVALID_GPA. This can then be trivially replaced if need be.

Back to this code, then we can have a slightly cleaner:

	unsigned long page_offset = kvm_is_error_gpa(gpa) ? offset_in_page(gpa) :
							    offset_in_page(uhva);


And I think it's worth asserting that exactly _one_ of GPA or HVA is valid, e.g.
to ensure KVM doesn't end up with botched offsets, and to make it a bit more
clear what's going on.


	if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva))
		return -EINVAL;

Sure.





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