[U-Boot] [PATCH 03/27] virtio: Add codes for virtual queue/ring management

[Tuomas Tynkkynen]

Hi Bin,

On 09/23/2018 04:42 PM, Bin Meng wrote:
> From: Tuomas Tynkkynen <tuomas.tynkkynen at iki.fi>
> 
> This adds support for managing virtual queue/ring, the channel
> for high performance I/O between host and guest.
> 
> Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen at iki.fi>
> Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
> ---
...

> +
> +/*
> + * Barriers in virtio are tricky. Since we are not in a hyperviosr/guest
> + * scenario, having these as nops is enough to work as expected.
> + */
> +
> +static inline void virtio_mb(void)
> +{
> +}
> +
> +static inline void virtio_rmb(void)
> +{
> +}
> +
> +static inline void virtio_wmb(void)
> +{
> +}
> +
> +static inline void virtio_store_mb(__virtio16 *p, __virtio16 v)
> +{
> +	WRITE_ONCE(*p, v);
> +}
> +
> +#endif /* _LINUX_VIRTIO_RING_H */
> 
I am not convinced it's safe to leave these barriers empty. For instance
if KVM acceleration is used, I believe U-Boot could be running on one
core concurrently with QEMU's IO thread running on a different core.
Thus we need memory barriers on both sides so that updates to the
vring are observed in the correct order.