Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

From: Christoph Hellwig
Date: Mon Feb 24 2020 - 12:17:02 EST


On Sat, Feb 22, 2020 at 02:07:58PM -0500, Michael S. Tsirkin wrote:
> On Fri, Feb 21, 2020 at 03:33:40PM +0100, Halil Pasic wrote:
> > AFAIU you have a positive attitude towards the idea, that
> > !F_VIRTIO_PLATFORM implies 'no DMA API is used by virtio'
> > should be scrapped.
> >
> > I would like to accomplish that without adverse effects to virtio-ccw
> > (because caring for virtio-ccw is a part of job description).
> >
> > Regards,
> > Halil
>
> It is possible, in theory. IIRC the main challenge is that DMA API
> has overhead of indirect function calls even when all it
> does it return back the PA without changes.

That overhead is gone now, the DMA direct calls are direct calls these
days.