From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: aliguori@us.ibm.com, aik@ozlabs.ru, rusty@rustcorp.com.au,
agraf@suse.de, qemu-devel@nongnu.org,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers
Date: Sun, 4 Sep 2011 11:47:50 +0300 [thread overview]
Message-ID: <20110904084750.GB18368@redhat.com> (raw)
In-Reply-To: <4E62538B.7030409@redhat.com>
On Sat, Sep 03, 2011 at 06:19:23PM +0200, Paolo Bonzini wrote:
> On 09/02/2011 05:45 PM, Michael S. Tsirkin wrote:
> >Well, can you describe an issue in virtio that lfence/sfence help solve
> >in terms of a memory model please?
> >Pls note that guest uses smp_ variants for barriers.
>
> /* Make sure buffer is written before we update index. */
> wmb();
>
> Without it, a guest could see a partially updated buffer, because
> the buffer and index writes are unlocked stores to different
> locations.
Sorry, I still don't understand. Yes, they are unlocked stores to different
locations. How does it follow that a guest could see a partially updated
buffer? Just to make sure - we are talking about x86 here, not ppc,
right?
> Even if the guest uses barriers, with ioeventfd it will only order
> the CPU that is running the guest, not the one that is running the
> iothread. In fact I'm surprised that it works at all under x86 with
> ioeventfd.
>
> Paolo
I can try to explain if I understand the problem you see.
--
MST
next prev parent reply other threads:[~2011-09-04 8:47 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-01 6:09 [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers David Gibson
2011-09-01 6:47 ` Paolo Bonzini
2011-09-02 0:08 ` David Gibson
2011-09-02 6:49 ` Paolo Bonzini
2011-09-03 11:53 ` Blue Swirl
2011-09-01 7:37 ` Sasha Levin
2011-09-01 7:38 ` Sasha Levin
2011-09-01 7:56 ` Paolo Bonzini
2011-09-02 0:09 ` David Gibson
2011-09-01 15:30 ` Michael S. Tsirkin
2011-09-01 16:14 ` Paolo Bonzini
2011-09-01 16:34 ` Michael S. Tsirkin
2011-09-01 20:31 ` Paolo Bonzini
2011-09-02 15:45 ` Michael S. Tsirkin
2011-09-03 14:46 ` David Gibson
2011-09-04 9:16 ` Michael S. Tsirkin
2011-09-05 4:43 ` David Gibson
2011-09-05 9:19 ` Michael S. Tsirkin
2011-09-06 3:12 ` David Gibson
2011-09-06 6:55 ` Paolo Bonzini
2011-09-06 9:02 ` David Gibson
2011-09-06 9:28 ` Avi Kivity
2011-09-06 9:35 ` Michael S. Tsirkin
2011-09-06 9:38 ` Paolo Bonzini
2011-09-05 7:41 ` Paolo Bonzini
2011-09-05 8:06 ` Michael S. Tsirkin
2011-09-05 9:42 ` Paolo Bonzini
2011-09-03 16:19 ` Paolo Bonzini
2011-09-04 8:47 ` Michael S. Tsirkin [this message]
2011-09-02 0:11 ` David Gibson
2011-09-02 6:11 ` Paolo Bonzini
2011-09-02 15:57 ` Michael S. Tsirkin
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