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

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Message-ID: <xhsmhy0wtngkd.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 25 Mar 2025 18:52:34 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Dave Hansen <dave.hansen@...el.com>, Jann Horn <jannh@...gle.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 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.