[PATCH] virtio: Try to untangle DMA coherency

[Alexander Graf]

On 02/01/2017 08:19 PM, Michael S. Tsirkin wrote:
> On Wed, Feb 01, 2017 at 06:27:09PM +0000, Will Deacon wrote:
>> On Wed, Feb 01, 2017 at 08:09:21PM +0200, Michael S. Tsirkin wrote:
>>> On Wed, Feb 01, 2017 at 12:25:57PM +0000, Robin Murphy wrote:
>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>>> index 7e38ed79c3fc..961af25b385c 100644
>>>> --- a/drivers/virtio/virtio_ring.c
>>>> +++ b/drivers/virtio/virtio_ring.c
>>>> @@ -20,6 +20,7 @@
>>>>  #include <linux/virtio_ring.h>
>>>>  #include <linux/virtio_config.h>
>>>>  #include <linux/device.h>
>>>> +#include <linux/property.h>
>>>>  #include <linux/slab.h>
>>>>  #include <linux/module.h>
>>>>  #include <linux/hrtimer.h>
>>>> @@ -160,10 +161,14 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
>>>>  		return true;
>>>>
>>>>  	/*
>>>> -	 * On ARM-based machines, the DMA ops will do the right thing,
>>>> -	 * so always use them with legacy devices.
>>>> +	 * On ARM-based machines, the coherent DMA ops will do the right
>>>> +	 * thing, so always use them with legacy devices. However, using
>>>> +	 * non-coherent DMA when the host *is* actually coherent, but has
>>>> +	 * forgotten to tell us, is going to break badly; since this situation
>>>> +	 * already exists in the wild, maintain the old behaviour there.
>>>>  	 */
>>>> -	if (IS_ENABLED(CONFIG_ARM) || IS_ENABLED(CONFIG_ARM64))
>>>> +	if ((IS_ENABLED(CONFIG_ARM) || IS_ENABLED(CONFIG_ARM64)) &&
>>>> +	    device_get_dma_attr(&vdev->dev) == DEV_DMA_COHERENT)
>>>>  		return !virtio_has_feature(vdev, VIRTIO_F_VERSION_1);
>>>>
>>>>  	return false;
>>>
>>> This is exactly what I feared.
>>
>> Yes, sorry about this. It works fine for virtio-pci (where "dma-coherent"
>> is used) and it also works on the fastmodel if you disable cache-modelling
>> (which is needed to make the thing run at a usable pace) so we didn't spot
>> this in testing.
>>
>>> Could we identify fastboot and do the special dance just for it?
>>
>> [assuming you mean fastmodel instead of fastboot]
>>
>>> I'd like to do that instead. It's fastboot doing the unreasonable thing
>>> here and deviating from what every other legacy device without exception
>>> did for years. If this means fastboot will need to update to virtio 1,
>>> all the better.
>>
>> The problem still exists with virtio 1, unless we require that the
>> "dma-coherent" property is set/unset correctly when VIRTIO_F_IOMMU_PLATFORM
>> is advertised by the device (which is what I suggested in my reply).
>
> I'm not ignoring that, but I need to understand that part a bit better.
> I'll reply to that patch in a day or two after looking at how _CCA is
> supposed to work.
>
>> We can't detect the fastmodel,
>
> Surely, it puts a hardware id somewhere? I think you mean
> fastmodel isn't always affected, right?
>
>> but we could implicitly treat virtio-mmio
>> devices as cache-coherent regardless of the "dma-coherent" flag. I already
>> prototyped this, but I suspect the devicetree people will push back (and
>> there's a similar patch needed for ACPI).
>>
>> See below. Do you prefer this approach?
>>
>> Will
>>
>> --->8
>
> I'd like to see basically
>
> if (fastmodel)
> 	a pile of special work-arounds
> else
> 	not less hacky but more common virtio work-arounds
>
> :)
>
> And then I can apply whatever comes from @arm.com and not
> worry about breaking actual hardware.

I'm actually seeing the exact same breakage in QEMU right now, so it's 
not fast model related at all. In QEMU we also don't properly set the 
dma-coherent flag, so we run into cache coherency problems.


Alex