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

From: Joerg Roedel <jroedel@suse.de>
To: Michael Roth <michael.roth@amd.com>
Cc: Peter Gonda <pgonda@google.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	the arch/x86 maintainers <x86@kernel.org>,
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	Marc Orr <marcorr@google.com>,
	Sathyanarayanan Kuppuswamy
	<sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH v12 32/46] x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers
Date: Fri, 11 Mar 2022 18:06:49 +0100	[thread overview]
Message-ID: <YiuBqZnjEUyMfBMu@suse.de> (raw)
In-Reply-To: <20220310212504.2kt6sidexljh2s6p@amd.com>

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.

Regards,

-- 
Jörg Rödel
jroedel@suse.de

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