linux-kernel - Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation

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Date:   Thu, 5 Oct 2023 22:20:47 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mika Penttilä <mpenttil@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Uros Bizjak <ubizjak@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation


* Brian Gerst <brgerst@...il.com> wrote:

> Looking at the compiled output, the only suboptimal code appears to be
> the canonical address test, where the C code uses the CL register for
> the shifts instead of immediates.
> 
>  180:   e9 00 00 00 00          jmp    185 <do_syscall_64+0x85>
>                         181: R_X86_64_PC32      .altinstr_aux-0x4
>  185:   b9 07 00 00 00          mov    $0x7,%ecx
>  18a:   eb 05                   jmp    191 <do_syscall_64+0x91>
>  18c:   b9 10 00 00 00          mov    $0x10,%ecx
>  191:   48 89 c2                mov    %rax,%rdx
>  194:   48 d3 e2                shl    %cl,%rdx
>  197:   48 d3 fa                sar    %cl,%rdx
>  19a:   48 39 d0                cmp    %rdx,%rax
>  19d:   75 39                   jne    1d8 <do_syscall_64+0xd8>

Yeah, it didn't look equivalent - so I guess we want a C equivalent for:

-       ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
-               "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57

instead of the pgtable_l5_enabled() runtime test that 
__is_canonical_address() uses?

Thanks,

	Ingo