Re: PCI cap for larger offsets/lengths

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

On Mon, Nov 26, 2018 at 03:51:45PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > So, I'm wondering whenever it makes sense to just do the same for your
> > > device.  Just use one pci bar as shared memory umbrella, specify that
> > > one using the virtio vendor cap, then have sub-regions within that bar
> > > for the various regions you have.  Manage them dynamically (using
> > > device-specific virtio commands) or just have a static configuration (in
> > > device-specific config space).
> > 
> > Ours are static subdivisions; so it felt easier to declare them; it's a
> > shame to make that device specific.
> 
> Shared memory handling is device specific anyway, so I fail to see why
> this is a problem.  Or do you want place virtio queues there (which
> could be common ground for multiple device types) ?
> 
> > > That avoids the problem with multiple capabilities of the same kind, and
> > > it also avoids exhausting the cap IDs quicky if every device defines
> > > their own VIRTIO_FOO_DEVICE_PCI_SHMCAP_ID_BAR_REGION.
> > 
> > Is having multiple capabilities of the same type actually a problem, or
> > is it just historical in the defitinition of virtio?
> 
> I think the reason is that you can in theory have the same region twice,
> once in an IO bar and once in an MMIO bar, and then the guest could
> prefer the IO bar if possible and use the MMIO bar otherwise (PCIe slot
> without IO address window for example).  I think that was never actually
> done in practice,

There's an option to enable that for AMD CPUs where MMIO
faults are slower than on intel CPUs.

> and for (prefetchable) memory bars it doesn't make
> sense at all.  So that would unlikely be a problem in practice.
> 
> Running out of capability IDs could become a real problem though.
> 
> cheers,
>   Gerd

We can always add more bits if we run out of these. there's
no real limit on capability size.

-- 
MST