Re: [virtio-dev] Re: [virtio] [PATCH] virtio-pci: access device using config

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/11/2013 07:27, Rusty Russell ha scritto:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > VIRTIO-39.
> We can discuss this on the call, but I don't like this.
> 
> PXE boot style environments will have to handle memory bars for modern
> PCIE devices.  Why should virtio be special?

Many devices these days do have some kind of backdoor to the memory
BARs, through either config space or an I/O BAR.  AHCI even standardizes
its backdoor with a special PCI capability.

GPUs also have backdoors in order to support accesses to BARs from the
VGA BIOS without knowing the allocation of the BARs.  Often the GPUs use
a register in the VGA ports (0x3b0...0x3df) so that they can even access
BARs correctly even if the OS has already accessed 0xcf8 but has not
accessed 0xcfc yet.

Paolo