linux-kernel - Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ

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Message-ID: <xhsmhv7rwnpax.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 26 Mar 2025 09:56:06 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Jann Horn <jannh@...gle.com>, Rik van Riel <riel@...riel.com>
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Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer
 flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

On 25/03/25 19:41, Jann Horn wrote:
> On Tue, Mar 25, 2025 at 6:52 PM Valentin Schneider <vschneid@...hat.com> wrote:
>> On 20/02/25 09:38, Dave Hansen wrote:
>> > But, honestly, I'm still not sure this is worth all the trouble. If
>> > folks want to avoid IPIs for TLB flushes, there are hardware features
>> > that *DO* that. Just get new hardware instead of adding this complicated
>> > pile of software that we have to maintain forever. In 10 years, we'll
>> > still have this software *and* 95% of our hardware has the hardware
>> > feature too.
>>
>> Sorry, you're going to have to deal with my ignorance a little bit longer...
>>
>> Were you thinking x86 hardware specifically, or something else?
>> AIUI things like arm64's TLBIVMALLE1IS can do what is required without any
>> IPI:
>>
>> C5.5.78
>> """
>> The invalidation applies to all PEs in the same Inner Shareable shareability domain as the PE that
>> executes this System instruction.
>> """
>>
>> But for (at least) these architectures:
>>
>>   alpha
>>   x86
>>   loongarch
>>   mips
>>   (non-freescale 8xx) powerpc
>>   riscv
>>   xtensa
>>
>> flush_tlb_kernel_range() has a path with a hardcoded use of on_each_cpu(),
>> so AFAICT for these the IPIs will be sent no matter the hardware.
>
> On X86, both AMD and Intel have some fairly recently introduced CPU
> features that can shoot down TLBs remotely.
>
> The patch series
> <https://lore.kernel.org/all/20250226030129.530345-1-riel@surriel.com/>
> adds support for the AMD flavor; that series landed in the current
> merge window (it's present in the mainline git repository now and should
> be part of 6.15). I think support for the Intel flavor has not yet
> been implemented, but the linked patch series mentions a plan to look
> at the Intel flavor next.

Thanks for the info!