diff mbox series

[v7,04/11] KVM: pfncache: base offset check on khva rather than gpa

Message ID 20231002095740.1472907-5-paul@xen.org
State New
Headers
Series KVM: xen: update shared_info and vcpu_info handling |

Commit Message

Paul Durrant Oct. 2, 2023, 9:57 a.m. UTC 
  From: Paul Durrant <pdurrant@amazon.com>

After a subsequent patch, the gpa may not always be set whereas khva will
(as long as the cache valid flag is also set).

No functional change intended.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
---
 virt/kvm/pfncache.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Comments

Sean Christopherson Oct. 31, 2023, 11:40 p.m. UTC | #1 
On Mon, Oct 02, 2023, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> After a subsequent patch, the gpa may not always be set whereas khva will
> (as long as the cache valid flag is also set).

This holds true only because there are no users of KVM_GUEST_USES_PFN, and
because hva_to_pfn_retry() rather oddly adds the offset to a NULL khva.

I think it's time to admit using this to map PFNs into the guest is a bad idea
and rip out KVM_GUEST_USES_PFN before fully relying on khva.

https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com
Paul Durrant Nov. 2, 2023, 6:11 p.m. UTC | #2 
On 31/10/2023 23:40, Sean Christopherson wrote:
> On Mon, Oct 02, 2023, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@amazon.com>
>>
>> After a subsequent patch, the gpa may not always be set whereas khva will
>> (as long as the cache valid flag is also set).
> 
> This holds true only because there are no users of KVM_GUEST_USES_PFN, and
> because hva_to_pfn_retry() rather oddly adds the offset to a NULL khva.
> 
> I think it's time to admit using this to map PFNs into the guest is a bad idea
> and rip out KVM_GUEST_USES_PFN before fully relying on khva.
> 
> https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com

Is this something you want me to fix?

   Paul
Sean Christopherson Nov. 3, 2023, 11:10 p.m. UTC | #3 
On Thu, Nov 02, 2023, Paul Durrant wrote:
> On 31/10/2023 23:40, Sean Christopherson wrote:
> > On Mon, Oct 02, 2023, Paul Durrant wrote:
> > > From: Paul Durrant <pdurrant@amazon.com>
> > > 
> > > After a subsequent patch, the gpa may not always be set whereas khva will
> > > (as long as the cache valid flag is also set).
> > 
> > This holds true only because there are no users of KVM_GUEST_USES_PFN, and
> > because hva_to_pfn_retry() rather oddly adds the offset to a NULL khva.
> > 
> > I think it's time to admit using this to map PFNs into the guest is a bad idea
> > and rip out KVM_GUEST_USES_PFN before fully relying on khva.
> > 
> > https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com
> 
> Is this something you want me to fix?

Yes?  I don't want to snowball your series, but I also really don't like the
confusion that is introduced by relying on khva while KVM_GUEST_USES_PFN is still
a thing.

Can you give it a shot, and then holler if it's a bigger mess than I'm anticipating?
I'm assuming/hoping it's a relatively small, one-off patch, but I haven't actually
dug through in-depth to figure out what all needs to change.
diff mbox series

Patch

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 17afbb464a70..37bcb4399780 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -83,15 +83,18 @@  bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
 	if (!gpc->active)
 		return false;
 
-	if ((gpc->gpa & ~PAGE_MASK) + len > PAGE_SIZE)
+	if (gpc->generation != slots->generation)
 		return false;
 
-	if (gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva))
+	if (kvm_is_error_hva(gpc->uhva))
 		return false;
 
 	if (!gpc->valid)
 		return false;
 
+	if (offset_in_page(gpc->khva) + len > PAGE_SIZE)
+		return false;
+
 	return true;
 }
 EXPORT_SYMBOL_GPL(kvm_gpc_check);