On Monday 27 April 2009 21:51:34 Michael S. Tsirkin wrote: > 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? Um, I think it should work too. > > > > 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? Now it haven't been allowed to do dynamically change...:( For now, please refer to assigned_device_update_msix_mmio in qemu/hw/device-assignment.c. It would scan the MSI-X page and transfer to KVM through ioctl. And for the kernel part, please refer to pci_enable_msix(). The userspace/kernel both still need change to support mask/unmask feature. That's still in TODO list(and I hope it can catch up with 2.6.31 merge window). -- regards Yang, Sheng > > 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, -- 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
