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

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.

Adam

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