Re: [PATCH v4 51/75] x86/sev-es: Handle MMIO events

Joerg Roedel
2020-07-22T07:55:53Z

Hi Mike,

On Tue, Jul 21, 2020 at 09:01:44PM +0000, Mike Stunes wrote:
> I’m running into an MMIO-related bug when I try testing this on our hypervisor.
>
> During boot, probe_roms (arch/x86/kernel/probe_roms.c) uses
> romchecksum over the video ROM and extension ROM regions. In my test
> VM, the video ROM romchecksum starts at virtual address
> 0xffff8880000c0000 and has length 65536. But, at address
> 0xffff8880000c4000, we switch from being video-ROM-backed to being
> unbacked by anything.
>
> With SEV-ES enabled, our platform handles reads and writes to unbacked
> memory by treating them as MMIO. So, the read from 0xffff8880000c4000
> causes a #VC, which is handled by do_early_exception.
>
> In handling the #VC, vc_slow_virt_to_phys fails for that address. My
> understanding is that the #VC handler should then add an entry to the
> page tables and retry the faulting access. Somehow, that isn’t
> happening. From the hypervisor side, it looks like the guest is
> looping somehow. (I think the VCPU is mostly unhalted and making
> progress, but the guest never gets past that romchecksum.) The guest
> never actually makes an MMIO vmgexit for that address.

That sounds like your guest is in a page-fault loop, but I can't yet
explain why. Can you please find out the instruction which is causing
the #VC exception?

If a page-fault happens during #VC emulation it is forwared to the
page-fault handler, so this should work. But somehow this isn't
happening or the page-fault handler can't map the faulting address.

Regards,

Joerg