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

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Date:   Fri, 11 Mar 2022 18:06:49 +0100
From:   Joerg Roedel <jroedel@...e.de>
To:     Michael Roth <michael.roth@....com>
Cc:     Peter Gonda <pgonda@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, linux-efi@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, Tony Luck <tony.luck@...el.com>,
        Marc Orr <marcorr@...gle.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH v12 32/46] x86/compressed/64: Add support for SEV-SNP
 CPUID table in #VC handlers

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@...e.de

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