linux-kernel - Re: [PATCH v14 11/13] x86/mm: do targeted broadcast flushing from tlbbatch code

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Message-ID: <Z8cdQwVnx8dmFDLA@google.com>
Date: Tue, 4 Mar 2025 15:33:23 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...el.com>, Rik van Riel <riel@...riel.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org, 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@....com, mingo@...nel.org
Subject: Re: [PATCH v14 11/13] x86/mm: do targeted broadcast flushing from
 tlbbatch code

On Tue, Mar 04, 2025 at 03:11:34PM +0100, Borislav Petkov wrote:
> On Tue, Mar 04, 2025 at 12:52:47PM +0000, Brendan Jackman wrote:
> > https://lore.kernel.org/all/CA+i-1C31TrceZiizC_tng_cc-zcvKsfXLAZD_XDftXnp9B2Tdw@mail.gmail.com/
> 
> Lemme try to understand what you're suggesting on that subthread:
> 
> > static inline void arch_start_context_switch(struct task_struct *prev)
> > {
> >     arch_paravirt_start_context_switch(prev);
> >     tlb_start_context_switch(prev);
> > }
> 
> This kinda makes sense to me...

Yeah so basically my concern here is that we are doing something
that's about context switching, but we're doing it in mm-switching
code, entangling an assumption that "context_switch() must either call
this function or that function". Whereas if we just call it explicitly
from context_switch() it will be much clearer.

> > Now I think about it... if we always tlbsync() before a context switch, is the
> > cant_migrate() above actually required? I think with that, even if we migrated
> > in the middle of e.g.  broadcast_kernel_range_flush(), we'd be fine? (At
> > least, from the specific perspective of the invplgb code, presumably having
> > preemption on there would break things horribly in other ways).
> 
> I think we still need it because you need to TLBSYNC on the same CPU you've
> issued the INVLPGB and actually, you want all TLBs to have been synched
> system-wide.
> 
> Or am I misunderstanding it?

Er, I might be exposing my own ignorance here. I was thinking that you
always go through context_switch() before you get migrated, but I
might not understand hwo migration happens.