Re: [EXTERNAL] Re: [svsm-devel] EDK2 CAA Page Fragmented Allocation

On Wed, May 21, 2025 at 09:16:07AM -0700, Adam Dunlap wrote:
> On Wed, May 21, 2025, 4:20 AM Gerd Hoffmann <[email protected]> wrote:
> > There is no trace of any explicit PVALIDATE calls.  So I'm wondering
> > what conditions might cause the VMM do actions like that?
> >
> > One possibly relevant detail is that the linux kernel and the uefi
> > runtime use different mappings and different virtual addresses to
> > access the caa page.
> 
> If the guest accesses a GPA with the c-bit disabled, then KVM will
> rmpupdate that page to shared. If the guest then accesses the same GPA
> with the c-bit enabled, then KVM will rmpupdate it back to private,
> but it will be unaccepted and thus give the 0x404 error. I've seen
> this happen while investigating several different bugs.

And this is what happened here too.  The kernel mapping had the c-bit,
the uefi mapping not.  With that fixed the kernel continues booting,
then traps into the next problem.  So more debugging ...

thanks & take care,
  Gerd