Cornelia Huck
2019-Apr-11 09:24 UTC
[RFC PATCH 10/12] virtio/s390: consolidate DMA allocations
On Wed, 10 Apr 2019 19:48:49 +0200 Halil Pasic <pasic at linux.ibm.com> wrote:> On Wed, 10 Apr 2019 18:36:43 +0200 > Cornelia Huck <cohuck at redhat.com> wrote: > > > On Wed, 10 Apr 2019 17:12:54 +0200 > > Halil Pasic <pasic at linux.ibm.com> wrote: > > > > > On Wed, 10 Apr 2019 10:46:49 +0200 > > > Cornelia Huck <cohuck at redhat.com> wrote: > > > > > > > On Fri, 5 Apr 2019 01:16:20 +0200 > > > > Halil Pasic <pasic at linux.ibm.com> wrote: > > > > > > > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c > > > > > index aa45a6a027ae..7268149f2ee8 100644 > > > > > --- a/drivers/s390/virtio/virtio_ccw.c > > > > > +++ b/drivers/s390/virtio/virtio_ccw.c > > > > > @@ -49,12 +49,12 @@ struct vq_config_block { > > > > > struct vcdev_dma_area { > > > > > unsigned long indicators; > > > > > unsigned long indicators2; > > > > > + struct vq_config_block config_block; > > > > > + __u8 status; /* TODO check __aligned(8); */ > > > > > > > > ...I think that needs attention. > > > > > > Yes I wanted to discuss this with you. I could not find anything > > > in the virtio spec that would put requirements on how this > > > status field needs to be aligned. But I did not look to hard. > > > > > > The ccw.cda can hold an arbitrary data address AFAIR (for indirect, > > > of course we do have alignment requirements). > > > > I think it needs to be doubleword aligned. > > > > I've re-read the part of the PoP that describes the ccw formats. And > it reinforced my position: for IDA and MIDA we need proper alignment, > but if the CCW ain't an indirect one there is no alignment requirement. > > QEMU also does not seem to check either. > > Can you double-check and provide me with a reference that proves me > wrong if I'm wrong.Ah, it was the ccw itself, not the cda. Indeed, there do not seem to be any requirements for direct addressing.> > > > > > > Apparently status used to be a normal field, and became a pointer with > > > 73fa21ea4fc6 "KVM: s390: Dynamic allocation of virtio-ccw I/O > > > data." (Cornelia Huck, 2013-01-07). I could not quite figure out why. > > > > In the beginning, the code used a below-2G-area for all commands. > > Rather than adding locking to avoid races there, that commit switches > > to allocating the needed structures individually. The status field > > needed to be below 2G, so it needed to be allocated separately. > > > > I get it now. The confusing part was that the field 'area' was about > holding the address of the also previously dynamically allocated > below 2G area that was used for talking to the hypervisor via CCW I/O. > > > > > > > So maybe dropping the TODO comment will do just fine. What do you think? > > > > I still think we just need to drop the comment, as we don't have to > align it.Agreed.