Re: [PATCH v1 2/6] xen/arm: Reserve resources for virtio-pci

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>]

On Tue, Sep 24, 2024 at 05:35:20PM +0100, Julien Grall wrote:
> Hi Edgar,
> 
> On 24/09/2024 17:23, Edgar E. Iglesias wrote:
> > From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
> > 
> > Reserve memory ranges and interrupt lines for an externally
> > emulated PCI controller (e.g by QEMU) dedicated to hosting
> > Virtio devices and potentially other emulated devices.
> > 
> > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
> > ---
> >   xen/include/public/arch-arm.h | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> > 
> > diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> > index e19f0251a6..654b827715 100644
> > --- a/xen/include/public/arch-arm.h
> > +++ b/xen/include/public/arch-arm.h
> > @@ -494,6 +494,20 @@ typedef uint64_t xen_callback_t;
> >   #define GUEST_RAM1_BASE   xen_mk_ullong(0x0200000000) /* 952GB of RAM @ 
> > 8GB */
> >   #define GUEST_RAM1_SIZE   xen_mk_ullong(0xee00000000)
> > +/* Virtio PCI - Ordered by decreasing size to keep things aligned */
> > +#define GUEST_VIRTIO_PCI_PREFETCH_MEM_TYPE  xen_mk_ullong(0x43000000)
> > +#define GUEST_VIRTIO_PCI_PREFETCH_MEM_BASE  xen_mk_ullong(0x0f000000000)
> > +#define GUEST_VIRTIO_PCI_PREFETCH_MEM_SIZE  xen_mk_ullong(0x100000000)
> > +
> > +#define GUEST_VIRTIO_PCI_ECAM_BASE      
> > (GUEST_VIRTIO_PCI_PREFETCH_MEM_BASE + \
> > +                                         
> > GUEST_VIRTIO_PCI_PREFETCH_MEM_SIZE)
> > +#define GUEST_VIRTIO_PCI_ECAM_SIZE      xen_mk_ullong(0x10000000)
> > +
> > +#define GUEST_VIRTIO_PCI_MEM_TYPE         xen_mk_ullong(0x02000000)
> > +#define GUEST_VIRTIO_PCI_MEM_BASE         (GUEST_VIRTIO_PCI_ECAM_BASE + \
> > +                                           GUEST_VIRTIO_PCI_ECAM_SIZE)
> > +#define GUEST_VIRTIO_PCI_MEM_SIZE         xen_mk_ullong(0x00002000000)
> 
> Why is this specific to virtio PCI? However, I am not entirely convinced we
> should have a second PCI hostbridge exposed to the guest for a few reasons:
>   1. This require to reserve yet another range in the address space (could
> be solved with a more dynamic layout)
>   2. From your instructions, the guest needs to explicitly do a PCI rescan.
> 
> So rather than having a second hostbridge, have you considered to extend the
> existing hostbridge (implemented in Xen) to support a mix of physical PCI
> device and virtual one?
>

Thanks Julien,

It's briefly come up in a couple of discussions but I haven't looked
carefully at it. It is a good idea and it's probably worth prototyping
to see what the gaps are in hypercall interfaces, QEMU support etc.

Cheers,
Edgar