Re: x86: missing FRED #PF event data?

[Sergey Senozhatsky <senozhatsky@chromium.org>]

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.