Re: [PATCH RFC] x86/syscall: Dispatch via sys_call_table when Spectre v2 and BHI are off

[Linus Torvalds]

On Thu, 29 Oct 2026 at 02:30, Breno Leitao <leitao@xxxxxxxxxx> wrote:
>
> With "x86_64/entry: Unconditionally use -fno-jump-table", x64_sys_call()
> becomes a compare tree on every x86_64 build, so system call dispatch is
> the same whether the mitigations are disabled at boot or at build time,
> which is basically my end goal.

You're basically saying "with that patch, both cases do equally badly
whether disabled at boot or build time".

That's not a good end goal. You want both cases to do equally *well*.

I do wonder if we could do computed jumps. It would be lovely if we
could basically do something like

      # syscall number in %rax
      leaq table(%rax,%rax,4),%rax
      jmp *%rax
table:
     jmp syscall0
     jmp syscall1
     jmp syscall2
    ....

and avoid any indirect memory lookup or indirect branches (outside of
the one computed one). And no extra call/ret frame.

It's possible that the branch prediction hardware hates those kinds of
very dense jmp patterns, though. I don't know.

We could have a single static jump that picks between this one and the
"conditional branch tree" version, as in your patch at the beginning
of the thread.

             Linus