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.
