Re: x86: missing FRED #PF event data?

[David Laight <david.laight.linux@gmail.com>]

On Mon, 10 Aug 2026 18:09:43 +0900
Sergey Senozhatsky <senozhatsky@chromium.org> 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.
> 

Is the same fault processed correctly from userspace?
If not it might be easier to test.

	David