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Message-ID: <ZR8an4+JbkLS8/Ol@gmail.com>
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
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