Re: [Qemu-devel] [PATCH v2 RFC] virtio-pci: fix LE host/BE guest capacity for blk

[Alexander Graf]

On 28.05.2013, at 12:18, Michael S. Tsirkin wrote:

> When a BE guest reads capacity from an LE host virtio-blk device or vice
> versa, it will get the dwords of the qword field swapped.
> As virtio-blk is the only one with such a quirk,
> and as non-pci transports don't do byte-swaps at all,
> solve this with a bit of device-specific hackery in
> virtio-pci.
> 
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
> 
> Changes from v1:
> fixed some obvious bugs.
> 
> I don't seem to be able to boot any big-endian
> guests ATM, so this is only compile-tested - sending

Huh? Why not? Qemu-system-ppc64 -M pseries -m 1G should be in good enough shape 
to boot your favorite Linux distro's installation DVD.

> this out for early feedback/flames.
> Testing reports also wellcome!

While the hack looks like a hack, I can't really think of anything better 
either.


Alex

> 
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 9668b2b..e7971cc 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -98,6 +98,12 @@
> /* HACK for virtio to determine if it's running a big endian guest */
> bool virtio_is_big_endian(void);
> 
> +#ifdef HOST_WORDS_BIGENDIAN
> +#define VIRTIO_HOST_IS_BIG_ENDIAN true
> +#else
> +#define VIRTIO_HOST_IS_BIG_ENDIAN false
> +#endif
> +
> static void virtio_pci_bus_new(VirtioBusState *bus, VirtIOPCIProxy *dev);
> 
> /* virtio device */
> @@ -411,6 +417,15 @@ static uint64_t virtio_pci_config_read(void *opaque, 
> hwaddr addr,
>         }
>         break;
>     case 4:
> +        /* Most devices don't have 64 bit config fields.
> +         * Block is an exception: first 8 bytes include
> +         * a 64 bit capacity field.
> +         */
> +        if (virtio_is_big_endian() != VIRTIO_HOST_IS_BIG_ENDIAN &&
> +            proxy->vdev->device_id == VIRTIO_ID_BLOCK && addr < 8) {
> +            /* Swap first two words */
> +            addr ^= 0x4;
> +        }
>         val = virtio_config_readl(proxy->vdev, addr);
>         if (virtio_is_big_endian()) {
>             val = bswap32(val);