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Re: [PATCH] x86/mm: slightly relax TLB-flush-local check again
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Fri, 29 Apr 2022 13:32:08 +0000
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- Thread-topic: [PATCH] x86/mm: slightly relax TLB-flush-local check again
On 29/04/2022 14:20, Jan Beulich wrote:
> system_state changes to SYS_STATE_smp_boot before alternative_branches()
> is invoked, yet that function, with CET-SS enabled, needs to invoke
> modify_xen_mappings(). Convert to check for the number of online CPUs,
> just like was done also in 88a037e2cfe1 / fa6dc0879ffd ("page_alloc:
> assert IRQs are enabled in heap alloc/free", both instance of which
> needed reverting for other reasons).
>
> Fixes: 78e072bc3750 ("x86/mm: avoid inadvertently degrading a TLB flush to
> local only")
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Only build-tested, as I don't have suitable hardware at hand.
I'll give it a test in just a moment, and while semantically I think
it's probably right, I don't think we want to express the logic like this.
num_online_cpus() is cpumask_weight(&cpu_online_map) behind the scenes
which is obnoxiously expensive for what we want.
For cases where we care just about UP vs SMP-ness, can't we just have an
bool which is re-evaluated each time we take a CPU online/offline? That
should be far lower overhead.
~Andrew
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