Messages in this thread | | | Date | Thu, 5 Oct 2023 22:20:47 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation |
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* Brian Gerst <brgerst@gmail.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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