linux-kernel - Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush

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Message-ID: <20170802081523.GB15219@arm.com>
Date:   Wed, 2 Aug 2017 09:15:23 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

On Wed, Aug 02, 2017 at 10:11:06AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 02, 2017 at 11:23:12AM +1000, Benjamin Herrenschmidt wrote:
> > On Wed, 2017-08-02 at 00:59 +0200, Peter Zijlstra wrote:
> > > > PowerPC for example uses PTESYNC before the TBLIE, so does a SYNC after
> > > > work? Ben?
> > > > From what I gather it is not. You have TLBSYNC for it. So the good news
> > 
> > tlbsync is pretty much a nop these days. ptesync is a strict superset
> > of sync and we have it after every tlbie.
> 
> In the radix code, yes. I got lost going through the hash code, and I
> didn't look at the 32bit code at all.
> 
> So the radix code does:
> 
>  PTESYNC
>  TLBIE
>  EIEIO; TLBSYNC; PTESYNC
> 
> which should be completely ordered against anything prior and anything
> following, and is I think the behaviour we want from TLB flushes in
> general, but is very much not provided by a number of architectures
> afaict.
> 
> Ah, found the hash-64 code, yes that's good too. The hash32 code lives
> in asm and confuses me, it has a bunch of SYNC, SYNC_601 and isync in.
> The nohash variant seems to do a isync after tlbwe, but again no clue.
> 
> 
> Now, do I go and attempt fixing all that needs fixing?
> 
> 
> x86 is good, our CR3 writes or INVLPG stuff is fully serializing.
> 
> arm is good, it does DSB ISH before and after
> 
> arm64 looks good too, although it plays silly games with the first
> barrier, but I trust that to be sufficient.

The first barrier only orders prior stores for us, because page table
updates are made using stores. A prior load could be reordered past the
invalidation, but can't make it past the second barrier.

I really think we should avoid defining TLB invalidation in terms of
smp_mb() because it's a lot more subtle than that.

Will