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

[Anthony Liguori <aliguori@us.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Tue, May 28, 2013 at 12:15:16PM -0500, Anthony Liguori wrote:
>> > @@ -455,6 +462,226 @@ static void virtio_pci_config_write(void *opaque, hwaddr addr,
>> >      }
>> >  }
>> >  
>> > +static uint64_t virtio_pci_config_common_read(void *opaque, hwaddr addr,
>> > +                                              unsigned size)
>> > +{
>> > +    VirtIOPCIProxy *proxy = opaque;
>> > +    VirtIODevice *vdev = proxy->vdev;
>> > +
>> > +    uint64_t low = 0xffffffffull;
>> > +
>> > +    switch (addr) {
>> > +    case offsetof(struct virtio_pci_common_cfg, device_feature_select):
>> > +        return proxy->device_feature_select;
>> 
>> Oh dear no...  Please use defines like the rest of QEMU.
>
> Any good reason not to use offsetof?
> I see about 138 examples in qemu.

There are exactly zero:

$ find . -name "*.c" -exec grep -l "case offset" {} \;
$

> Anyway, that's the way Rusty wrote it in the kernel header -
> I started with defines.
> If you convince Rusty to switch I can switch too,

We have 300+ devices in QEMU that use #defines.  We're not using this
kind of thing just because you want to copy code from the kernel.

>> https://github.com/aliguori/qemu/commit/587c35c1a3fe90f6af0f97927047ef4d3182a659
>> 
>> And:
>> 
>> https://github.com/aliguori/qemu/commit/01ba80a23cf2eb1e15056f82b44b94ec381565cb
>> 
>> Which lets virtio-pci be subclassable and then remaps the config space to
>> BAR2.
>
>
> Interesting. Have the spec anywhere?

Not yet, but working on that.

> You are saying this is going to conflict because
> of BAR2 usage, yes?

No, this whole thing is flexible.  I had to use BAR2 because BAR0 has to
be the vram mapping.  It also had to be an MMIO bar.

The new layout might make it easier to implement a device like this.  I
shared it mainly because I wanted to show the subclassing idea vs. just
tacking an option onto the existing virtio-pci code in QEMU.

Regards,

Anthony Liguori

> So let's only do this virtio-fb only for new layout, so we don't need
> to maintain compatibility. In particular, we are working
> on making memory BAR access fast for virtio devices
> in a generic way. At the moment they are measureably slower
> than PIO on x86.
>
>
>> Haven't looked at the proposed new ring layout yet.
>> 
>> Regards,
>
> No new ring layout. It's new config layout.
>
>
> -- 
> MST
> --
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