linux-kernel - Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()

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Message-ID: <1552405610.3083.17.camel@HansenPartnership.com>
Date:   Tue, 12 Mar 2019 08:46:50 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     David Miller <davem@...emloft.net>, hch@...radead.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com,
        linux-arm-kernel@...ts.infradead.org, linux-parisc@...r.kernel.org
Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through
 vmap()

On Tue, 2019-03-12 at 07:54 -0400, Michael S. Tsirkin wrote:
> On Tue, Mar 12, 2019 at 03:17:00PM +0800, Jason Wang wrote:
> > 
> > On 2019/3/12 上午11:52, Michael S. Tsirkin wrote:
> > > On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
[...]
> > At least for -stable, we need the flush?
> > 
> > 
> > > Three atomic ops per bit is way to expensive.
> > 
> > 
> > Yes.
> > 
> > Thanks
> 
> See James's reply - I stand corrected we do kunmap so no need to
> flush.

Well, I said that's what we do on Parisc.  The cachetlb document
definitely says if you alter the data between kmap and kunmap you are
responsible for the flush.  It's just that flush_dcache_page() is a no-
op on x86 so they never remember to add it and since it will crash
parisc if you get it wrong we finally gave up trying to make them.

But that's the point: it is a no-op on your favourite architecture so
it costs you nothing to add it.

James