On Tue, Jun 11, 2013 at 10:53:48AM +0300, Gleb Natapov wrote: > On Tue, Jun 11, 2013 at 10:10:47AM +0300, Michael S. Tsirkin wrote: > > On Thu, Jun 06, 2013 at 10:02:14AM -0500, Anthony Liguori wrote: > > > Gleb Natapov <gleb@xxxxxxxxxx> writes: > > > > > > > On Wed, Jun 05, 2013 at 07:41:17PM -0500, Anthony Liguori wrote: > > > >> "H. Peter Anvin" <hpa@xxxxxxxxx> writes: > > > >> > > > >> > On 06/05/2013 03:08 PM, Anthony Liguori wrote: > > > >> >>> > > > >> >>> Definitely an option. However, we want to be able to boot from native > > > >> >>> devices, too, so having an I/O BAR (which would not be used by the OS > > > >> >>> driver) should still at the very least be an option. > > > >> >> > > > >> >> What makes it so difficult to work with an MMIO bar for PCI-e? > > > >> >> > > > >> >> With legacy PCI, tracking allocation of MMIO vs. PIO is pretty straight > > > >> >> forward. Is there something special about PCI-e here? > > > >> >> > > > >> > > > > >> > It's not tracking allocation. It is that accessing memory above 1 MiB > > > >> > is incredibly painful in the BIOS environment, which basically means > > > >> > MMIO is inaccessible. > > > >> > > > >> Oh, you mean in real mode. > > > >> > > > >> SeaBIOS runs the virtio code in 32-bit mode with a flat memory layout. > > > >> There are loads of ASSERT32FLAT()s in the code to make sure of this. > > > >> > > > > Well, not exactly. Initialization is done in 32bit, but disk > > > > reads/writes are done in 16bit mode since it should work from int13 > > > > interrupt handler. The only way I know to access MMIO bars from 16 bit > > > > is to use SMM which we do not have in KVM. > > > > > > Ah, if it's just the dataplane operations then there's another solution. > > > > > > We can introduce a virtqueue flag that asks the backend to poll for new > > > requests. Then SeaBIOS can add the request to the queue and not worry > > > about kicking or reading the ISR. > > > > This will pin a host CPU. > > If we do something timer based it will likely > > both increase host CPU utilization and slow device down. > > > > If we didn't care about performance at all we could > > do config cycles for signalling, which is much > > more elegant than polling in host, but I don't think > > that's the case. > > > I wouldn't call BIOS int13 interface performance critical. So the plan always was to - add an MMIO BAR - add a register for pci-config based access to devices hpa felt performance does matter there but didn't clarify why ... > > > > > > SeaBIOS is polling for completion anyway. > > > > I think that's different because a disk will normally respond > > quickly. So it polls a bit, but then it stops as > > there are no outstanding requests. > > > > -- > > MST > > -- > Gleb. -- 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
