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Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + co
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments |
Date: |
Wed, 11 Nov 2009 21:56:16 +0200 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Thu, Nov 12, 2009 at 02:37:26AM +0800, Scott Tsai wrote:
> On Thu, Nov 12, 2009 at 2:09 AM, Michael S. Tsirkin <address@hidden> wrote:
> >> I do have a newbie question, when exactly would vrtio have to handle
> >> concurrent access from multiple threads?
> >> My current reading of the code suggests:
> >> 1. when CONFIG_IOTHREAD is true
> >> 2. when CONFIG_KVM is true and the guest machine has multiple CPUs
> >
> > Right. I don't think CONFIG_IOTHREAD can work correctly
> > without kvm though: how would atomics be handled?
>
> I naively imagined it to work like this:
> When CONFIG_IOTHREAD is true and CONFIG_KVM is false,
> all the tcg CPUs run in the tcg_cpu_thread and device emulation code
> runs in io_thread,
> so if the tcg translators generate suitable memory barrier
> instructions when it sees a "lfence", "sfence", "mfence" instruction
> while emulating a x86 or "sync" while emulating a MIPS everything
> should work
that might not be enough. guest can do e.g. atomics on the same memory
with iothread. In parctice, with virtio it doesn't.
> but a quick look at target-*/translate.c suggests memory
> barrier instructions are treated as nops.
>
> So maybe --enable-io-thread while --disable-kvm should not be allowed
> at configure time.
> Does anyone actually ship qemu with CONFIG_IOTHREAD enabled?
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, (continued)
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Paul Brook, 2009/11/11
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Paul Brook, 2009/11/11
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Paul Brook, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Scott Tsai, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Scott Tsai, 2009/11/11
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments,
Michael S. Tsirkin <=
- Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Jamie Lokier, 2009/11/12
- [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments, Michael S. Tsirkin, 2009/11/11