linux-kernel - Re: [PATCH v3 23/75] x86/boot/compressed/64: Setup GHCB Based VC Exception handler

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Message-ID: <20200513085928.GB4025@zn.tnic>
Date:   Wed, 13 May 2020 10:59:28 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <jroedel@...e.de>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Mike Stunes <mstunes@...are.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 23/75] x86/boot/compressed/64: Setup GHCB Based VC
 Exception handler

On Tue, May 12, 2020 at 11:08:12PM +0200, Joerg Roedel wrote:
> No, this is a recent addition, otherwise this breaks out-of-tree builds
> (make O=/some/path ...) because inat-tables.c (included from inat.c) is
> generated during build and ends up in the $(objtree).

Please add a blurb about this above it otherwise no one would have a
clue why it is there.

> Because the immediate is the last part of the instruction which is
> decoded (even if there is no immediate). The .got field is set when
> either the immediate was decoded successfully or, in case the
> instruction has no immediate, when the rest of the instruction was
> decoded successfully. So testing immediate.got is the indicator whether
> decoding was successful.

Hm, whether the immediate was parsed correctly or it wasn't there and
using that as the sign whether the instruction was decoded successfully
sounds kinda arbitrary.

@Masami: shouldn't that insn_get_length() thing return a value to denote
whether the decode was successful or not instead of testing arbitrary
fields?

Pasting for you the code sequence again:

	...
        insn_get_length(&ctxt->insn);

        ret = ctxt->insn.immediate.got ? ES_OK : ES_DECODE_FAILED;
	...

I wonder if one would be able to do instead:

	if (insn_get_length(&ctxt->insn))
		return ES_OK;

	return ES_DECODE_FAILED;

to have it straight-forward...

> Probably better rename the function, it also does the sev-es protocol
> version negotiation and all other related setup tasks. Maybe
> sev_es_setup() is a better name?

Sure.

> If the above call fails its the end of the systems lifetime, because we
> can't continue to boot an SEV-ES guest when we have no GHCB.
> 
> BUG_ON() and friends are also not available in the pre-decompression
> boot stage.

Oh ok, error() does hlt the system.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette