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Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device
From: |
Rusty Russell |
Subject: |
Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device |
Date: |
Tue, 18 Dec 2007 13:31:51 +1100 |
User-agent: |
KMail/1.9.6 (enterprise 0.20070907.709405) |
On Wednesday 12 December 2007 12:40:43 Anthony Liguori wrote:
> Rusty Russell wrote:
> > On Sunday 09 December 2007 09:02:48 Anthony Liguori wrote:
> >> If QEMU ever got true SMP support, then virtio would not work as it
> >> requires 16-bit atomic writes which AFAIK is not possible on a number of
> >> non-x86 architectures.
> >
> > Hmm? Where is this requirement coming from?
> >
> > I think everyone should stop using the word "atomic" in virtio
> > discussions; it's confusing.
>
> The virtio ring queue indices are 16-bit and are readable to one end
> while writable on the other end. To ensure that this can be done in a
> lock-less way, it's necessary to atomically update the index. Atomic is
> the right word here because if the 16-bit write gets converted into two
> 8-bit writes, then very bad things could happen with SMP.
Of course, but that's insane. Your assertion that it's not possible on a
number of non-x86 architectures is what I'm questioning here. You're
confusing the inability of architectures to atomically *modify* a 16 bit
value and our requirement, where even if you found an architecture which
couldn't do 16 bit writes, you can do it as a 32 bit write.
Hope that clarifies,
Rusty.
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, (continued)
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Anthony Liguori, 2007/12/05
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Jamie Lokier, 2007/12/06
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Paul Brook, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Jamie Lokier, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Paul Brook, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Blue Swirl, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Jamie Lokier, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Anthony Liguori, 2007/12/08
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Rusty Russell, 2007/12/11
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Anthony Liguori, 2007/12/11
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device,
Rusty Russell <=
- Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device, Anthony Liguori, 2007/12/08