Re: [PATCH v4 3/3] vhost/vsock: keep IOTLB across feature updates

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Aug 14, 2026 at 03:29:03PM +0800, Jia Jia wrote:
>VHOST_SET_FEATURES is also used to update logging while a device is
>running. When ACCESS_PLATFORM stays enabled, allocating a new empty
>IOTLB on every call drops valid translations and forces avoidable
>misses.
>
>Initialize the device IOTLB only when one does not already exist.
>
>Fixes: e13a6915a03f ("vhost/vsock: add IOTLB API support")
>Signed-off-by: Jia Jia <physicalmtea@gmail.com>
>---
> drivers/vhost/vsock.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index b69c260eaeff..9142fa1143b2 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -869,6 +869,7 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
> 	    vsock->dev.iotlb)
> 		vhost_clear_device_iotlb(&vsock->dev);
>

Ah, okay, now I see why you did it in the other patch, but if we move 
dev.iotlb checks in the functions we can still do that.

>-	if ((features & (1ULL << VIRTIO_F_ACCESS_PLATFORM))) {
>+	if ((features & (1ULL << VIRTIO_F_ACCESS_PLATFORM)) &&
>+	    !vsock->dev.iotlb) {
> 		if (vhost_init_device_iotlb(&vsock->dev))

IIUC vhost_init_device_iotlb() handles the case where dev.iotlb is 
already initialized and override it. Here we are preventing that.

Will we do the same in vhost-net? If yes, all the callers will call 
vhost_init_device_iotlb() with `dev.iotlb` NULL, so should we remove 
that behaviour and add the check we are adding here inside the 
vhost_init_device_iotlb()?

Or there is an use case where vhost_init_device_iotlb() is called with 
an already initialized dev.iotlb ?

Thanks,
Stefano

> 			goto err;
>-- 
>2.34.1
>