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Date:   Tue, 20 Oct 2020 17:48:12 +0200
From:   Joerg Roedel <jroedel@suse.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Joerg Roedel <joro@8bytes.org>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Martin Radev <martin.b.radev@gmail.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/5] x86/boot/compressed/64: Check SEV encryption in
 64-bit boot-path
Message-ID: <20201020154812.GB22179@suse.de>
References: <20201020121856.19427-1-joro@8bytes.org>
 <20201020121856.19427-4-joro@8bytes.org>
 <20201020141259.GC2996696@rani.riverdale.lan>
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On Tue, Oct 20, 2020 at 10:12:59AM -0400, Arvind Sankar wrote:
> On Tue, Oct 20, 2020 at 02:18:54PM +0200, Joerg Roedel wrote:
> Why use r10-r12 rather than the caller-save registers? Even for the head
> code where you need to perserve the cr3 value you can just return it in
> rax?

It can surely be optimized, but it makes the code less robust.  This
function is only called from assembly so the standard x86-64 calling
conventions might not be followed strictly. I think its better to make
as few assumptions as possible about the calling code to avoid
regressions. Changes to the head code are not necessarily tested with
SEV/SEV-ES guests by developers.

Regards,

	Joerg