linux-kernel - Re: [PATCH v2 3/5] x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path

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Message-ID: <20201020160428.GA3233355@rani.riverdale.lan>
Date:   Tue, 20 Oct 2020 12:04:28 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Martin Radev <martin.b.radev@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] x86/boot/compressed/64: Check SEV encryption in
 64-bit boot-path

On Tue, Oct 20, 2020 at 05:48:12PM +0200, Joerg Roedel wrote:
> 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

This is called from both assembly and C, but anyway, you're already
assuming r10 and r11 can be clobbered safely, and you just took out the
save/restores in set_sev_encryption_mask, which is actually called only
from assembly.