Re: [RFC PATCH 02/12] virtio/s390: DMA support for virtio-ccw

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 02/12] virtio/s390: DMA support for virtio-ccw
Date: Tue, 9 Apr 2019 17:47:55 +0200	[thread overview]
Message-ID: <20190409174755.5f62d04e.cohuck@redhat.com> (raw)
In-Reply-To: <20190409152313.0296e8f1@oc2783563651>

On Tue, 9 Apr 2019 15:23:13 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Tue, 9 Apr 2019 15:01:20 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Tue, 9 Apr 2019 13:29:27 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> > > On Tue, 9 Apr 2019 11:57:43 +0200
> > > Cornelia Huck <cohuck@redhat.com> wrote:
> > >   
> > > > On Fri,  5 Apr 2019 01:16:12 +0200
> > > > Halil Pasic <pasic@linux.ibm.com> wrote:

> > > > > @@ -1255,6 +1254,18 @@ static int virtio_ccw_online(struct ccw_device *cdev)
> > > > >  		ret = -ENOMEM;
> > > > >  		goto out_free;
> > > > >  	}
> > > > > +	vcdev->vdev.dev.parent = &cdev->dev;    
> > > > 
> > > > That one makes sense, pci and mmio are doing that as well.
> > > >     
> > > > > +	cdev->dev.dma_mask = &vcdev->dma_mask;    
> > > > 
> > > > That one feels a bit weird. Will this change in one of the follow-on
> > > > patches? (Have not yet looked at the whole series.)    
> > > 
> > > I don't thinks so. Do you mean this should happen within the cio code?
> > > I think I started out with the idea to keep the scope as narrow as
> > > possible. Do you have any suggestions?  
> > 
> > From what I see, you set the mask from the virtio-ccw side, then
> > propagate it up to the general ccw_device, and then the generic virtio
> > code will fetch it from the ccw_device.  
> 
> Right! For some reason dma_mask is a pointer. And I need virtio core to
> use a sane value for virtio_ccw devices.
> 
> > Don't you potentially need
> > something for other ccw_devices in that protected hipervisor case as
> > well (e.g for 3270)?  
> 
> 
> Maybe, maybe not. The first stage is likely to be virito only. I would
> prefer sorting out stuff like 3270 as the need arises. Also see my
> response to patch 4 (Message-Id: <20190409141114.7dcce94a@oc2783563651>).

As long as the infrastructure is flexible enough to be extended later,
ok. I still need to read that mail.

> 
> >   
> > >   
> > > >     
> > > > > +
> > > > > +	ret = dma_set_mask_and_coherent(&cdev->dev,
> > > > > DMA_BIT_MASK(64));
> > > > > +	if (ret)
> > > > > +		ret = dma_set_mask_and_coherent(&cdev->dev,
> > > > > +
> > > > > DMA_BIT_MASK(32));
> > > > > +	if (ret) {
> > > > > +		dev_warn(&cdev->dev, "Failed to enable 64-bit
> > > > > or 32-bit DMA.  Trying to continue, but this might not
> > > > > work.\n");    
> > > > 
> > > > This does not look like you'd try to continue?
> > > >     
> > > 
> > > I remember now. First I did continue, then I changed this to fail
> > > hard so I can not ignore any such problems while smoke testing ('I
> > > don't always check the kernel messages'), but kept the old message.
> > > This basically should not fail anyway, otherwise we have a problem
> > > AFAIU.
> > > 
> > > By the way virtio-pci tries to continue indeed, and this is also
> > > where the wording comes from ;).
> > > 
> > > What would you prefer? Try to continue or fail right away?  
> > 
> > If it does not have a chance of working properly in the general case,
> > I'd fail.
> >   
> 
> Agreed! I will make it so. Would dropping '  Trying to continue, but
> this might not work.' from the warning message work for you?

Sounds fine.

> 
> I could also drop the attempt to set a 32 bit mask if you agree. Do you?

Only if you also drop it from the message as well ;)

Not sure in what cases you'll fail to set a 64 bit mask, but succeed
with a 32 bit mask. If there's no sensible situation where that might
happen, I'd just go ahead and drop it.