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

[Breno Leitao]

Hello Peter,

On Thu, Oct 29, 2026 at 11:49:38AM +0100, Peter Zijlstra wrote:
> On Thu, Oct 29, 2026 at 03:27:23AM -0700, Breno Leitao wrote:
> Bah, this reminds me I still have these patches:
> 
>  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/syscall
> 
> which make your patch impossible. The reason for not dispatching through
> the syscall table is that syscall hijacking becomes a _LOT_ more
> difficult.

Cool, this would in fact solve my problem as well.

My goal is to make booting with mitigations=off equivalent to building
with CONFIG_CPU_MITIGATIONS=n, so we can ship a single kernel and pick
the mitigations at boot time. Your patches get there in a different way,
but that doesn't matter for my use case.

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.

That said, can I do anything to help those patches land? I see they are
from June, and I am wondering if anything is blocking them.

Also, KBUILD_RISTFLAGS in that patch looks like a typo for
KBUILD_RUSTFLAGS.

Thanks,
--breno