On Mon, Apr 27, 2009 at 09:16:14PM +0800, Sheng Yang wrote: > On Monday 27 April 2009 18:41:17 Michael S. Tsirkin wrote: > > Sheng, Marcelo, > > I've been reading code in qemu/hw/device-assignment.c, and > > I have a couple of questions about msi-x implementation: > > Hi Michael > > > 1. What is the reason that msix_table_page is allocated > > with mmap and not with e.g. malloc? > > msix_table_page is a page, and mmap allocate memory on page boundary. So I use > it. Just wondering, would e.g. posix_memalign work here as well? > > 2. msix_table_page has the guest view of the msix table for the device. > > However, even this memory isn't mapped into guest directly, instead > > msix_mmio_read/msix_mmio_write perform the write in qemu. > > Won't it be possible to map this page directly into > > guest memory, reducing the overhead for table writes? > > First, Linux configured the real MSI-X table in device, which is out of our > scope. KVM accepted the interrupt from Linux, then inject it to the guest > according to the MSI-X table setting of guest. So KVM should know about the > page modification. For example, MSI-X table got mask bit which can be written > by guest at any time(this bit haven't been implement yet, but should be soon), > then we should mask the correlated vector of real MSI-X table; then guest may > modified the MSI address/data, that also should be intercepted by KVM and used > to update our knowledge of guest. So we can't passthrough the modification. Right, I see that. However all msix_mmio_write does is a memcpy. So what I don't understand yet, what causes the real MSI-X table to be modified? Where's that code? > If guest can write to the real device MSI-X table directly, it would cause > chaos on interrupt delivery, for what guest see is totally different with > what's host see... Obviously. Thanks, -- MST -- 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
