"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > On Wed, Jun 05, 2013 at 03:42:57PM -0500, Anthony Liguori wrote: >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: >> >> Can you explain? I thought the whole trick with separating out the >> virtqueue notification register was to regain the performance? > > Yes but this trick only works well with NPT (it's still a bit > slower than PIO but not so drastically). > Without NPT you still need a page walk so it will be slow. Do you mean NPT/EPT? If your concern is shadow paging, then I think you're concerned about hardware that is so slow to start with that it's not worth considering. >> >> It also maps to what regular hardware does. I highly doubt that there >> >> are any real PCI cards that made the shift from PCI to PCI-e without >> >> bumping at least a revision ID. >> > >> > Only because the chance it's 100% compatible on the software level is 0. >> > It always has some hardware specific quirks. >> > No such excuse here. >> > >> >> It also means we don't need to play games about sometimes enabling IO >> >> bars and sometimes not. >> > >> > This last paragraph is wrong, it ignores the issues 3) to 5) >> > I added above. >> > >> > If you do take them into account: >> > - there are reasons to add MMIO BAR to PCI, >> > even without PCI express >> >> So far, the only reason you've provided is "it doesn't work on some >> architectures." Which architectures? > > PowerPC wants this. Existing PowerPC remaps PIO to MMAP so it works fine today. Future platforms may not do this but future platforms can use a different device. They certainly won't be able to use the existing drivers anyway. Ben, am I wrong here? >> > - we won't be able to drop IO BAR from virtio >> >> An IO BAR is useless if it means we can't have more than 12 devices. > > > It's not useless. A smart BIOS can enable devices one by one as > it tries to boot from them. A smart BIOS can also use MMIO to program virtio. Regards, Anthony Liguori -- 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
