linux-kernel - Re: [PATCH] x86/sev: Don't use cc_platform

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Message-ID: <YwZyjE6BmnvxHXcR@zn.tnic>
Date:   Wed, 24 Aug 2022 20:48:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Asish Kalra <ashish.kalra@....com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        stable@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/sev: Don't use cc_platform_has() for early SEV-SNP
 calls

On Wed, Aug 24, 2022 at 11:43:10AM -0700, Dave Hansen wrote:
> So, we don't have *ANY* control over where the compiler uses jump
> tables.  The kernel just happened to add some code that uses them, fell
> over, and this adds a hack to get booting again.
> 
> Isn't this a bigger problem?

I had the same question already. Was thinking of maybe disabling
the compiler from producing jump tables in the ident-mapped code.
Tom's argument is that that might prevent the compiler from doing
optimizations but I haven't talked to compiler folks whether those
optimizations are even worth the effort.

Regardless, the potential problem is limited:

"# (jump-tables are implicitly disabled by RETPOLINE)"

i.e., only RETPOLINE=n builds for now which should be a minority?

I guess when this explodes somewhere else again, we will have to
generalize a fix.

Thx.

-- 
Regards/Gruss,
    Boris.

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