Re: [PATCH 1/2] hw/nvme: Implement shadow doorbell buffer support

[John Levon]

On Thu, Jun 09, 2022 at 08:29:30AM -0600, Keith Busch wrote:

> On Wed, Jun 08, 2022 at 10:55:30PM +0200, Klaus Jensen wrote:
> > 
> > Keith, is this a bug in the kernel? If the code here would expect the
> > doorbell buffer to be updated for the admin queue as well, would we
> > break?
> 
> The admin queue has to be created before db buffer can be set up, so we can't
> rely on it. And this is a performance feature. Admin commands have never been
> considered performant, so we decided not to consider it though the spec allows
> it.

It's not just unnecessary, but enabling shadow doorbells on admin queues will
actively break device implementations (such as SPDK), which have had to presume
that driver implementations don't use shadow doorbells for the admin queue.

regards
john