Re: [PATCH v4 0/3] Adjust the output of x-query-virtio-status

[Michael S. Tsirkin]

On Wed, Mar 13, 2024 at 10:40:21AM +0100, Markus Armbruster wrote:
> I could be awkward for the use case described in PATCH 1's commit
> message:
> 
>     However, we sometimes want to compare features and status bits without
>     caring for their exact meaning.  Say we want to verify the correctness
>     of the virtio negotiation between guest, QEMU, and OVS-DPDK.  We can use
>     QMP command x-query-virtio-status to retrieve vhost-user net device
>     features, and the "ovs-vsctl list interface" command to retrieve
>     interface features.  Without commit f3034ad71fc, we could then simply
>     compare the numbers.  With this commit, we first have to map from the
>     strings back to the numeric encoding.

So, consider how guest kernel presents features then. Do you happen to know?
It's actually a binary string:

static ssize_t features_show(struct device *_d,
                             struct device_attribute *attr, char *buf)
{
        struct virtio_device *dev = dev_to_virtio(_d);
        unsigned int i;
        ssize_t len = 0;

        /* We actually represent this as a bitstring, as it could be
         * arbitrary length in future. */
        for (i = 0; i < sizeof(dev->features)*8; i++)
                len += sysfs_emit_at(buf, len, "%c",
                               __virtio_test_bit(dev, i) ? '1' : '0');
        len += sysfs_emit_at(buf, len, "\n");
        return len;
}
static DEVICE_ATTR_RO(features);

-- 
MST