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

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Message-ID: <20250211110721.GF29593@noisy.programming.kicks-ass.net>
Date: Tue, 11 Feb 2025 12:07:21 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Brendan Jackman <jackmanb@...gle.com>
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 Fri, Feb 07, 2025 at 03:28:31PM +0100, Brendan Jackman wrote:
> Hi Rik,
> 
> On Thu, 6 Feb 2025 at 05:45, Rik van Riel <riel@...riel.com> wrote:
> > This is done because 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.
> >
> > Always handle page table freeing with MMU_GATHER_RCU_TABLE_FREE.
> > Using RCU synchronization between page table freeing and get_user_pages_fast()
> > allows bare metal to also do TLB flushing while interrupts are disabled.
> >
> > Various places in the mm do still block IRQs or disable preemption
> > as an implicit way to block RCU frees.
> >
> > That makes it safe to use INVLPGB on AMD CPUs.
> 
> 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.

- 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.