Re: x86: missing FRED #PF event data?

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 10, 2026, Peter Zijlstra wrote:
> On Mon, Aug 10, 2026 at 06:09:43PM +0900, Sergey Senozhatsky wrote:
> > On (26/08/10 11:03), Peter Zijlstra wrote:
> > > > > > and got the following while running my tests (and well, we don't crash
> > > > > > anymore):
> > > > > > 
> > > > > > [  254.040223] :: fixed up address to ffff9c4d64af4000  [[fred: 0 cr2: ffff9c4d64af4000]]
> > > > > > ...
> > > > > > [ 1821.904563] :: fixed up address to ffff9c4e9dd0a000  [[fred: 0 cr2: ffff9c4e9dd0a000]]
> > > > > > 
> > > > > > Does any of this make sense to you?
> > > > > 
> > > > > I think the explanation is some pKVM shenanigans.  Sorry for the noise.
> > > > 
> > > > No, I think we are back at square one.  I thought that maybe pKVM
> > > > was disabling FRED and that was causing issues.  But I actually see
> > > > that both cpu_feature_enabled(X86_FEATURE_FRED) and (cr4 & X86_CR4_FRED)
> > > > claim FRED is enabled, yet fred #PF data is 0 while CR2 holds the correct
> > > > address.
> > > 
> > > Can you reproduce on real hardware? I mean, you were babbling about pKVM
> > > (whatever that is), so I'm thinking you're on some dodgy virt thing.
> > > 
> > > Not that I have any actual FRED hardware to test anything on :/
> > 
> > It's an actual H/W, a laptop (Intel(R) Core(TM) Ultra 7 355 powered),
> > I was babbling about pKVM because it seems that the best way to
> > reproduce is to launch a VM, yet the crash is happening on the host
> > side.
> 
> Urg, ok, lets wait for hpa to wake up. He's the one that knows most
> about FRED.

But to Peter's point, babbling about pKVM suggests that you're running out-of-tree
patches to enable pKVM on x86, which means this could be a bug somewhere in those
out-of-tree patches.  And if you're indeed running pKVM, it also probably means
the "host" is deprivileged into a VM, which brings KVM and virtualization back
into the mix as possible suspects.

So, is this happening on a kernel that's fairly close to a vanilla upstream, or
is it indeed happening on a pKVM-capable kernel?