linux-kernel - Re: [PATCH v9 01/12] x86/mm: make MMU_GATHER_RCU_TABLE

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Message-ID: <CA+i-1C1fP+Lk4FGO+-sLMq+LmjVjm-1r-wG15oKtDDu0=gi+Gg@mail.gmail.com>
Date: Tue, 11 Feb 2025 13:10:25 +0100
From: Brendan Jackman <jackmanb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>, x86@...nel.org, linux-kernel@...r.kernel.org, 
	bp@...en8.de, dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com, 
	nadav.amit@...il.com, thomas.lendacky@....com, kernel-team@...a.com, 
	linux-mm@...ck.org, akpm@...ux-foundation.org, jannh@...gle.com, 
	mhklinux@...look.com, andrew.cooper3@...rix.com, 
	Manali Shukla <Manali.Shukla@....com>
Subject: Re: [PATCH v9 01/12] x86/mm: make MMU_GATHER_RCU_TABLE_FREE unconditional

On Tue, 11 Feb 2025 at 12:07, Peter Zijlstra <peterz@...radead.org> wrote:
> > It would be nice to update the CONFIG_MMU_GATHER_RCU_TABLE_FREE
> > comment in mm/mmu_gather.c to mention INVLPG alongside "Architectures
> > that do not have this (PPC)"
>
> Why? This is just one more architecture that does broadcast.

Hmm yeah, I didn't really make the leap from "doesn't do an IPI" to
"that just means it uses broadcast TLB flush". In that light I can see
how it's "just another architecture".

I do think it would make sense to be more explicit about that, even
though it seems obvious now you point it out. But it's not really
relevant to this patchset.

> - and while that's being updated it would
> > also be useful to note down the paravirt thing you explained above,
> > IMO it's pretty helpful to have more examples of the concrete usecases
> > for this logic.
>
> Look at kvm_flush_tlb_multi() if you're interested. The notable detail
> is that is avoids flushing TLB for vCPUs that are preempted, and instead
> lets the vCPU resume code do the invalidate.

Oh that actually looks like a slightly different case from what Rik mentioned?

> some paravirt TLB flush implementations
> handle the TLB flush in the hypervisor, and will do the flush
> even when the target CPU has interrupts disabled.

Do we have

a) Systems where the flush gets completely pushed into the hypervisor
- xen_flush_tlb_multi()?

b) Systems where the guest coordinates with the hypervisor to avoid
IPI-ing preempted vCPUs?

Maybe I can send a separate patch to note this in the commentary, it's
interesting and useful to know.