Re: [PATCH v3 7/9] vhost: Add new UAPI to support change to task mode

[Mike Christie <michael.christie@oracle.com>]

On 11/5/24 3:39 AM, Jason Wang wrote:
> On Tue, Nov 5, 2024 at 3:28 PM Cindy Lu <lulu@redhat.com> wrote:
>>
>> Add a new UAPI to enable setting the vhost device to task mode.
>> The userspace application can use VHOST_SET_INHERIT_FROM_OWNER
>> to configure the mode if necessary.
>> This setting must be applied before VHOST_SET_OWNER, as the worker
>> will be created in the VHOST_SET_OWNER function
>>
>> Signed-off-by: Cindy Lu <lulu@redhat.com>
>> ---
>>  drivers/vhost/vhost.c      | 15 ++++++++++++++-
>>  include/uapi/linux/vhost.h |  2 ++
>>  2 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index c17dc01febcc..70c793b63905 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2274,8 +2274,9 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>>  {
>>         struct eventfd_ctx *ctx;
>>         u64 p;
>> -       long r;
>> +       long r = 0;
>>         int i, fd;
>> +       bool inherit_owner;
>>
>>         /* If you are not the owner, you can become one */
>>         if (ioctl == VHOST_SET_OWNER) {
>> @@ -2332,6 +2333,18 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>>                 if (ctx)
>>                         eventfd_ctx_put(ctx);
>>                 break;
>> +       case VHOST_SET_INHERIT_FROM_OWNER:
>> +               /*inherit_owner can only be modified before owner is set*/
>> +               if (vhost_dev_has_owner(d))
>> +                       break;
>> +
>> +               if (copy_from_user(&inherit_owner, argp,
>> +                                  sizeof(inherit_owner))) {
>> +                       r = -EFAULT;
>> +                       break;
>> +               }
>> +               d->inherit_owner = inherit_owner;
>> +               break;
> 
> Is there any case that we need to switch from owner back to kthread?
> If not I would choose a more simplified API that is just
> VHOST_INHERIT_OWNER.

I can't think of any need to be able to switch back and forth for
general use.

However for this patchset, I think in patch 9/9 we set the default as:

inherit_owner_default = true

so the default is to use vhost_tasks.

With that code, we would need VHOST_SET_INHERIT_FROM_OWNER so userspace
can set the kernel to use kthreads.

I'm not sure if in the past emails it was resolved what the default would
be.