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

[Julien Grall <julien@xxxxxxx>]

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?
Cheers,

--
Julien Grall