[RFC PATCH v6 27/29] x86/mm/pti: Implement a TLB flush immediately after a switch to kernel CR3

[Valentin Schneider]

On 29/10/25 11:31, Frederic Weisbecker wrote:
> Le Wed, Oct 29, 2025 at 11:16:23AM +0100, Valentin Schneider a écrit :
>> On 28/10/25 16:59, Frederic Weisbecker wrote:
>> > Le Fri, Oct 10, 2025 at 05:38:37PM +0200, Valentin Schneider a écrit :
>> >> @@ -171,8 +172,27 @@ For 32-bit we have the following conventions - kernel is built with
>> >>      andq    $(~PTI_USER_PGTABLE_AND_PCID_MASK), \reg
>> >>  .endm
>> >>
>> >> -.macro COALESCE_TLBI
>> >> +.macro COALESCE_TLBI scratch_reg:req
>> >>  #ifdef CONFIG_COALESCE_TLBI
>> >> +	/* No point in doing this for housekeeping CPUs */
>> >> +	movslq  PER_CPU_VAR(cpu_number), \scratch_reg
>> >> +	bt	\scratch_reg, tick_nohz_full_mask(%rip)
>> >> +	jnc	.Lend_tlbi_\@
>> >
>> > I assume it's not possible to have a static call/branch to
>> > take care of all this ?
>> >
>>
>> I think technically yes, but that would have to be a per-cpu patchable
>> location, which would mean something like each CPU having its own copy of
>> that text page... Unless there's some existing way to statically optimize
>>
>>   if (cpumask_test_cpu(smp_processor_id(), mask))
>>
>> where @mask is a boot-time constant (i.e. the nohz_full mask).
>
> Or just check housekeeping_overriden static key before everything. This one is
> enabled only if either nohz_full, isolcpus or cpuset isolated partition (well,
> it's on the way for the last one) are running, but those are all niche, which
> means you spare 99.999% kernel usecases.
>

Oh right, if NOHZ_FULL is actually in use.

Yeah that housekeeping key could do since, at least for the cmdline
approach, it's set during start_kernel(). I need to have a think about the
runtime cpuset case.

Given we have ALTERNATIVE's in there I assume something like a
boot-time-driven static key could do, but I haven't found out yet if and
how that can be shoved in an ASM file.

> Thanks.
>
>>
>> > Thanks.
>> >
>> > --
>> > Frederic Weisbecker
>> > SUSE Labs
>>
>>
>
> --
> Frederic Weisbecker
> SUSE Labs