Re: [PATCH v12 32/46] x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers

Peter Gonda
2022-03-14T17:34:56Z

On Fri, Mar 11, 2022 at 10:06 AM Joerg Roedel <jroedel@suse.de> wrote:
>
> On Thu, Mar 10, 2022 at 03:25:04PM -0600, Michael Roth wrote:
> > Joerg, do you have more background on that? Would it make sense, outside
> > of this series, to change it to a terminate? Maybe with a specific set
> > of error codes for ES_{OK,UNSUPPORTED,VMM_ERROR,DECODE_FAILED}?
>
> This seems to be a left over from development of the SEV-ES guest
> patch-set. I wanted to see whether the VM crashed due to a triple fault
> or an error in the #VC handler. The halt loop can be replaced by
> termination request now.
>
> > > I am still working on why the early_printk()s in that function are not
> > > working, it seems that they lead to a different halt.
> >
> > I don't see a different halt. They just don't seem to print anything.
> > (keep in mind you still need to advance the IP or else the guest is
> > still gonna end up spinning here, even if you're removing the halt loop
> > for testing purposes)
>
> The early_printks() also cause #VC exceptions, and if that handling is
> broken for some reason nothing will be printed.
>
> >
> > > working, it seems that they lead to a different halt. Have you tested
> > > any of those error paths manually? For example if you set your CPUID
> > > bits to explicitly fail here do you see the expected printks?
> >
> > I think at that point in the code, when the XSAVE stuff is setup, the
> > console hasn't been enabled yet, so messages would get buffered until they
> > get flushed later (which won't happen since there's halt loop after). I
> > know in some cases devs will dump the log buffer from memory instead to get
> > at the error messages for early failures. (Maybe that's also why Joerg
> > decided to use a halt loop there instead of terminating?)
>
> It is hard to dump the log-buffer from encrypted memory :) But I
> remember having seen messages from these early_printks under SEV-ES for
> different bugs. Not sure why they don't appear in this situation.
>
> > So maybe reworking the error handling in handle_vc_boot_ghcb() to use
> > sev_es_terminate() might be warranted, but probably worth checking with
> > Joerg first, and should be done as a separate series since it is not
> > SNP-related.
>
> I am fine with this change.

I'll send a patch out for that.

I was also thinking about adding a vcpu run exit reason for
termination. It would be nice to get a more informative exit reason
than -EINVAL in userspace. Thoughts?

>
> Regards,
>
> --
> Jörg Rödel
> jroedel@suse.de
>
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5
> 90409 Nürnberg
> Germany
>
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Ivo Totev
>