Re: [RFC PATCH 10/12] virtio/s390: consolidate DMA allocations

From: Cornelia Huck <cohuck@redhat.com>
To: Halil Pasic <pasic@linux.ibm.com>
Cc: kvm@vger.kernel.org, linux-s390@vger.kernel.org,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Sebastian Ott <sebott@linux.ibm.com>,
	virtualization@lists.linux-foundation.org,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Viktor Mihajlovski <mihajlov@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Farhan Ali <alifm@linux.ibm.com>,
	Eric Farman <farman@linux.ibm.com>
Subject: Re: [RFC PATCH 10/12] virtio/s390: consolidate DMA allocations
Date: Thu, 11 Apr 2019 11:24:46 +0200	[thread overview]
Message-ID: <20190411112446.013cad09.cohuck@redhat.com> (raw)
In-Reply-To: <20190410194849.511ecc46@oc2783563651>

On Wed, 10 Apr 2019 19:48:49 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Wed, 10 Apr 2019 18:36:43 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Wed, 10 Apr 2019 17:12:54 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> > > On Wed, 10 Apr 2019 10:46:49 +0200
> > > Cornelia Huck <cohuck@redhat.com> wrote:
> > >   
> > > > On Fri,  5 Apr 2019 01:16:20 +0200
> > > > Halil Pasic <pasic@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.