Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 06, 2013 at 10:02:14AM -0500, Anthony Liguori wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > On Wed, Jun 05, 2013 at 07:41:17PM -0500, Anthony Liguori wrote:
> >> "H. Peter Anvin" <hpa@zytor.com> 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.

> 
> 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