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[Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio
From: |
Paul Brook |
Subject: |
[Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio |
Date: |
Wed, 23 Dec 2009 13:32:53 +0000 |
User-agent: |
KMail/1.12.4 (Linux/2.6.32-trunk-amd64; KDE/4.3.4; x86_64; ; ) |
On Wednesday 23 December 2009, Michael S. Tsirkin wrote:
> On Wed, Dec 23, 2009 at 12:25:46PM +0000, Paul Brook wrote:
> > > > > So possibly this means that we
> > > > > could optimize the barrier away, but I don't think this amounts to
> > > > > a serious issue, I guess portability/readability is more important.
> > > >
> > > > The more important issue is that regular devices which to not require
> > > > coherency or ordering can omit this lock.
> > >
> > > So let them. What's the issue?
> >
> > The issue is that at you're only fixing half the problem. Barriers aren't
> > sufficient to get the semantics you need. You also need atomicity.
> >
> > Given we need both, why not actually defined an API that gives you this?
>
> Because, I do not want to define APIs, I want to reuse an existing one.
Except that, say you said later in your email, no API exists for doing atomic
accesses, so you need different code anyway.
> E.g. what is stw_barrier? atomic read followed by a barrier? read
> preceded by a barrier? and what kind of barrier? IMO
stw_barrier is an ordered atomic store.
>With KVM, even these partial barriers add small but measureable overhead
>(about 2%).
Now are you measuring that overhead? How much additional overhead does a full
barrier incur?
Paul
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, (continued)
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Anthony Liguori, 2009/12/22
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Paul Brook, 2009/12/22
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/22
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Paul Brook, 2009/12/22
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Paul Brook, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Paul Brook, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio,
Paul Brook <=
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Paul Brook, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23
- Re: [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Avi Kivity, 2009/12/22
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Rusty Russell, 2009/12/23
- [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio, Michael S. Tsirkin, 2009/12/23