Re: [Qemu-devel] [PATCH v2 02/22] virtio: run drivers in 32bit mode

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

On Tue, Jun 30, 2015 at 10:38:53AM +0200, Gerd Hoffmann wrote:
> virtio version 1.0 registers can (and actually do in the qemu
> implementation) live in mmio space.  So we must run the blk and
> scsi virtio drivers in 32bit mode, otherwise we can't access them.
> 
> This also allows to drop a bunch of GET_LOWFLAT calls from the virtio
> code in the following patches.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Is there an advantage to running them in a 16 bit mode?

If yes - it's actually possible to access virtio in 16 bit mode because
all virtio devices have a special capability which maps memory into pci
config space. So you can poke memory (slowly) through the cf8/cfc
registers.

It's missing in QEMU but that's a clear spec violation: spec
says it's mandatory.

I'll code it up and send an RFC shortly.


> ---
>  src/block.c         | 8 +++++---
>  src/hw/virtio-blk.c | 2 +-
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/src/block.c b/src/block.c
> index 3f7ecb1..a9b9851 100644
> --- a/src/block.c
> +++ b/src/block.c
> @@ -503,8 +503,10 @@ process_op(struct disk_op_s *op)
>      case DTYPE_CDEMU:
>          ret = process_cdemu_op(op);
>          break;
> -    case DTYPE_VIRTIO_BLK:
> -        ret = process_virtio_blk_op(op);
> +    case DTYPE_VIRTIO_BLK: ;
> +        extern void _cfunc32flat_process_virtio_blk_op(void);
> +        ret = call32(_cfunc32flat_process_virtio_blk_op
> +                     , (u32)MAKE_FLATPTR(GET_SEG(SS), op), DISK_RET_EPARAM);
>          break;
>      case DTYPE_AHCI: ;
>          extern void _cfunc32flat_process_ahci_op(void);
> @@ -526,7 +528,6 @@ process_op(struct disk_op_s *op)
>          break;
>      case DTYPE_USB:
>      case DTYPE_UAS:
> -    case DTYPE_VIRTIO_SCSI:
>      case DTYPE_LSI_SCSI:
>      case DTYPE_ESP_SCSI:
>      case DTYPE_MEGASAS:
> @@ -534,6 +535,7 @@ process_op(struct disk_op_s *op)
>          break;
>      case DTYPE_USB_32:
>      case DTYPE_UAS_32:
> +    case DTYPE_VIRTIO_SCSI:
>      case DTYPE_PVSCSI: ;
>          extern void _cfunc32flat_scsi_process_op(void);
>          ret = call32(_cfunc32flat_scsi_process_op
> diff --git a/src/hw/virtio-blk.c b/src/hw/virtio-blk.c
> index e2dbd3c..15ac171 100644
> --- a/src/hw/virtio-blk.c
> +++ b/src/hw/virtio-blk.c
> @@ -77,7 +77,7 @@ virtio_blk_op(struct disk_op_s *op, int write)
>      return status == VIRTIO_BLK_S_OK ? DISK_RET_SUCCESS : DISK_RET_EBADTRACK;
>  }
>  
> -int
> +int VISIBLE32FLAT
>  process_virtio_blk_op(struct disk_op_s *op)
>  {
>      if (! CONFIG_VIRTIO_BLK)
> -- 
> 1.8.3.1