linux-kernel - Re: [PATCH v9 10/12] x86/mm: do targeted broadcast flushing from tlbbatch code

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Message-ID: <CA+i-1C2CcQmrbdv0wLUJFD6m85D7LekejU=VdgMxnqZW=DRPxA@mail.gmail.com>
Date: Wed, 12 Feb 2025 11:38:38 +0100
From: Brendan Jackman <jackmanb@...gle.com>
To: Rik van Riel <riel@...riel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de, 
	peterz@...radead.org, 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 10/12] x86/mm: do targeted broadcast flushing from
 tlbbatch code

On Tue, 11 Feb 2025 at 21:21, Rik van Riel <riel@...riel.com> wrote:
>
> On Tue, 2025-02-11 at 11:02 +0100, Brendan Jackman wrote:
> >
> > So I think here we're encoding the assumption that context_switch()
> > always calls either enter_lazy_tlb() or switch_mm_irqs_off(), which
> > is
> > a little awkward, plus the job of these functions is already kinda
> > hazy and this makes it even hazier. What about doing it in
> > arch_start_context_switch() instead?
> >
> > That would mean a bit of plumbing since we'd still wanna have the
> > tlbsync() in tlb.c, but that seems worth it to me. Plus, having it in
> > one place would give us a spot to add a comment. Now that you point
> > it
> > out it does indeed seem obvious but it didn't seem so yesterday.
> >
> While that would be a little bit cleaner to maintain,
> in theory, I'm not convinced that adding an extra
> function call to the context switch path is worthwhile
> for that small maintenance benefit.

I don't see why it has to introduce a function call, can't we just
have something like:

static inline void arch_start_context_switch(struct task_struct *prev)
{
    arch_paravirt_start_context_switch(prev);
    tlb_start_context_switch(prev);
}

The paravirt one would disappear when !CONFIG_PARAVIRT (as the current
arch_start_context_switch() does) and the tlb one would disappear when
!CONFIG_X86_BROADCAST_TLB_FLUSH. The whole thing should be inlinable.