Re: x86: missing FRED #PF event data?

[Peter Zijlstra]

On Mon, Aug 10, 2026 at 05:58:18PM +0900, Sergey Senozhatsky wrote:
> On (26/08/10 16:38), Sergey Senozhatsky wrote:
> > [..]
> > > All the crashes are reported as NULL ptr derefs, however, I believe this
> > > is not exactly the case.  In all crashes CR2 is 0x1000 aligned (we always
> > > crash accessing first byte of a page).  It seems that csum_partial() calls
> > > load_unaligned_zeropad() and we hit what load_unaligned_zeropad() comment
> > > describes as very unlikely) case: "word being a page-crosser and the
> > > next page not being mapped").  So instead of reading 4 remaining bytes
> > > of the page and zeroes for trailing 4 bytes, we panic().  It appears that
> > > FRED #PF is set to 0 while CR2 points to a correct page address.  I added
> > > a simple printk to exc_page_fault:
> > > 
> > >         address = cpu_feature_enabled(X86_FEATURE_FRED) ? fred_event_data(regs) : read_cr2();
> > >         /* Fall back to CR2 if FRED event data was empty */
> > >         if (unlikely(!address)) {
> > >                 address = read_cr2();
> > >                 pr_err(":: fixed up address to %lx [[fred: %lx cr2: %lx]]\n", address, fred_event_data(regs), read_cr2());
> > >         }
> > > 
> > > 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 :/