linux-kernel - Re: [RFC PATCH v4 6/8] x86/mm: use RAR for kernel TLB flushes

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Message-ID: <7997802c151577c5e681813e121ded3e0d84d3ec.camel@surriel.com>
Date: Sat, 28 Jun 2025 21:30:23 -0400
From: Rik van Riel <riel@...riel.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
 bp@...en8.de, 	x86@...nel.org, nadav.amit@...il.com, seanjc@...gle.com,
 tglx@...utronix.de, 	mingo@...nel.org, Rik van Riel <riel@...com>
Subject: Re: [RFC PATCH v4 6/8] x86/mm: use RAR for kernel TLB flushes

On Fri, 2025-06-27 at 16:27 +0300, Kirill A. Shutemov wrote:
> On Thu, Jun 19, 2025 at 04:03:58PM -0400, Rik van Riel wrote:
> > From: Rik van Riel <riel@...com>
> > 
> > Use Intel RAR for kernel TLB flushes, when enabled.
> > 
> > Pass in PCID 0 to smp_call_rar_many() to flush the specified
> > addresses,
> > regardless of which PCID they might be cached under in any
> > destination CPU.
> > 
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > ---
> >  arch/x86/mm/tlb.c | 38 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 38 insertions(+)
> > 
> > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > index 39f80111e6f1..8931f7029d6c 100644
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -21,6 +21,7 @@
> >  #include <asm/apic.h>
> >  #include <asm/msr.h>
> >  #include <asm/perf_event.h>
> > +#include <asm/rar.h>
> >  #include <asm/tlb.h>
> >  
> >  #include "mm_internal.h"
> > @@ -1468,6 +1469,18 @@ static void do_flush_tlb_all(void *info)
> >  	__flush_tlb_all();
> >  }
> >  
> > +static void rar_full_flush(const cpumask_t *cpumask)
> > +{
> > +	guard(preempt)();
> > +	smp_call_rar_many(cpumask, 0, 0, TLB_FLUSH_ALL);
> > +	invpcid_flush_all();
> 
> I don't follow why do we need to call invpcid_flush_all() here in
> addition
> to smp_call_rar_many(). Hm?
> 
We shouldn't have to.

Once we figure out why the RAR flush isn't
working right (despite the RAR transitioning
from RAR_PENDING to RAR_SUCCESS) we should be
able to get rid of this call.


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