[Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio

[Michael S. Tsirkin]

On Tue, Dec 22, 2009 at 04:25:32PM +0000, Paul Brook wrote:
> On Tuesday 22 December 2009, Anthony Liguori wrote:
> > On 12/22/2009 05:26 AM, Michael S. Tsirkin wrote:
> > > On Tue, Dec 08, 2009 at 06:18:18PM +0200, Michael S. Tsirkin wrote:
> > >> The following fixes a class of long-standing bugs in qemu:
> > >> when kvm is enabled, guest might access device structures
> > >> in memory while they are updated by qemu on another CPU.
> > >> In this scenario, memory barriers are necessary to prevent
> > >> host CPU from reordering memory accesses, which might confuse
> > >> the guest.
> > >>
> > >> This patch only fixes virtio, but other emulated devices
> > >> might have a similar bug. They'll need to be discovered
> > >> and addressed case by case.
> 
> Real devices generally aren't cache coherent, so I'd expect problems to be 
> rare. I guess theoretically you may need barriers around the MMIO/IO port 
> handlers, though in practice the KVM context switch probably provides this 
> anyway.
> 
> > >> This is still under test ... meanwhile: any early feedback/flames?
> > >
> > > Any comments on this one?
> > > The patch works fine in my testing, and even though
> > > it did not fix a crash that I hoped it will fix,
> > > it seems required for correctness... Right?
> > 
> > It's definitely better than what we have.  Rusty mentioned something to
> > me a bit ago about the barriers for virtio in the kernel needing some
> > work.  I've been meaning to ask him about it in the context of this patch.
> 
> Given this is supposed to be portable code, I wonder if we should have atomic 
> ordered memory accessors instead.
> 
> Paul

Could you clarify please?

The infiniband bits I used as base are very portable,
I know they build on a ton of platforms. I just stripped
a couple of infiniband specific assumptions from there.

Do you suggest we use __sync_synchronize?
Unfortunately this is broken or slow on many platforms.
I do use it when it seems safe or when we see a platform
we don't know about.

-- 
MST