Messages in this thread | | | From | Brian Gerst <> | Date | Thu, 5 Oct 2023 11:13:16 -0400 | Subject | Re: [PATCH v2 0/6] x86: Clean up fast syscall return validation |
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On Thu, Oct 5, 2023 at 4:22 AM Ingo Molnar <mingo@kernel.org> wrote: > > > * Brian Gerst <brgerst@gmail.com> wrote: > > > This patch set cleans up the tests done to determine if a fast syscall > > return instruction can be used to return to userspace. It converts the > > code to C, and refactors existing code to be more readable. > > > > v2: > > - Fix shift value for canonical RIP test and use > > __is_canonical_address() > > > > Brian Gerst (6): > > x86/entry/64: Remove obsolete comment on tracing vs. SYSRET > > x86/entry/64: Convert SYSRET validation tests to C > > x86/entry/compat: Combine return value test from syscall handler > > x86/entry/32: Convert do_fast_syscall_32() to bool return type > > x86/entry/32: Remove SEP test for SYSEXIT > > x86/entry/32: Clean up syscall fast exit tests > > > > arch/x86/entry/common.c | 99 +++++++++++++++++++++----------- > > arch/x86/entry/entry_32.S | 2 +- > > arch/x86/entry/entry_64.S | 68 +--------------------- > > arch/x86/entry/entry_64_compat.S | 12 ++-- > > arch/x86/include/asm/syscall.h | 6 +- > > 5 files changed, 77 insertions(+), 110 deletions(-) > > Ok, so I've applied patches #1, #3, #4 and #5 to tip:x86/entry, > (ie. skipped #2 & #6 for now), as they look correct and are good > improvements. None of these four patches depend on the skipped > patches in some way I missed, correct? > > As for #2, I looked at the before/after disassembly, and the new > C code in do_syscall_64() looked suboptimal on x86-64 defconfig, > if I was reading it right. > > Mind re-evaluating that, and if you still think the C conversion > is a good idea, mind putting a before/after analysis of the > generated instructions into the changelog? This is our primary > system call return path after all.
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>
Was there anything else specifically that you can point out?
Brian Gerst
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