Re: [PATCH V7 v7] virtio: introduce SUSPEND bit in device status

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 13, 2024 at 07:58:32AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, August 13, 2024 1:22 PM
> > 
> Removing intel email id again to avoid constant bouncing messages.

Thanks.

> > On Tue, Aug 13, 2024 at 04:42:06AM +0000, Parav Pandit wrote:
> > > a. suspending a device is non frequent operation (in order of N
> > operations/sec, where N is roughly in range of 10 or 100) per device?
> > > b. A software-based device may not always want to force VM_EXIT on read
> > and write on the device_status register?
> > 
> > I just see where this is going.  Parav, if you want a device with as little as
> > possible memory on it, using DMA as much as possible, what you want is
> > develop an alternative transport that will let you control status without using
> > memory accesses.
> No, there is no need of DMA here.
> This is not about less memory either.

OK, sorry. I misunderstood.

> As I explained, the challenge is implementing suspend bits to operate at sub-microsecond level granularity.

Yes, I think the proposal is too vague on how all this happens.
Replied separately.

> > This will also, with time, allow doing this for existing config space.
> > We aren't blocking all spec development until you do, and the tradeoff of
> > easy to debug, robust MMIO control versus complex but maybe cheaper for
> > some hardware vendors DMA based access is up to the device.
> >
> No one is blocking the spec etc. Not sure why you say that.
> The idea is to have suspend as command register, and its status to reflect in the existing device_status.
> 
> Last time you recommended to only make design suggestion and not go down the exact details of the bit and let author to fix it.
> 
> If you prefer to discuss the bit level details, it looks good to me to go to that details.

Sorry, reacted too early.

> > --
> > MST