> > Also, an unrelated questions: With PIO, requests were ordered, which > > means that if we wrote to the queue selector and then read from a > > queue register we would read the correct queue info. > > Is the same thing assured to us with MMIO? > > For real PCI, reads do not bypass writes in PCI. However this > is only true if both are MMIO or both PIO reads. > I don't think the ordering of MMIO versus PIO is guaranteed. > > On KVM, the kernel doesn't do anything to guarantee ordering. > So you get the natural ordering of the CPU. > > > If we write to a queue > > selector and immediately read from queue info would we be reading the > > right info, or is there the slight chance that it would get reordered > > and we would be reading queue info first and writing to the selector > > later? > > The thing to realize is that write to queue selector with KVM is in the > end performed by host. And reading queue info means that host will be > reading the queue selector. So this is a write followed by read > from the same address. AFAIK no CPUs can reorder such accesses, > so you get the right info. (As far as I understand all the complexity ;-) Memory mapped using ioremap()-like functions should preserve access order. In case of ARM architecture such memory region is defined (at the page tables level) as a "device memory" (contrary to "normal memory") and the processor will not try to be clever about it. I know next-to-nothing about x86, but I suppose similar idea applies. Cheers! Paweł -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
