Subject: [PATCH] virtio: Try to untangle DMA coherency

From: [email-masked] (Michael S. Tsirkin)
Date: Wed, 1 Feb 2017 21:19:22 +0200

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.

> >From f6ad4e331c26e7ba53132c8cc74e26f782391570 Mon Sep 17 00:00:00 2001
> From: Will Deacon <[email-masked]>
> Date: Mon, 30 Jan 2017 17:28:31 +0000
> Subject: [PATCH] of/address: Allow devices to report DMA coherency based on
>  compatible string
> 
> Some devices (e.g. virtio-mmio) are implicitly cache coherent with respect
> to DMA operations and therefore do not mandate the use of "dma-coherent"
> in their devicetree bindings. In order to ensure that these devices work
> correctly when using the DMA API, we need to treat them specially in
> of_dma_is_coherent by identifying them as unconditionally coherent.
> 
> This patch adds a static, table-based search against the compatible
> string for the device in of_dma_is_coherent before walking the
> hierarchy looking for "dma-coherent". This allows existing virtio-mmio
> devices (e.g. those emulated by QEMU) to function correctly when placed
> behind an IOMMU that requires use of the DMA ops to map the vring.
> 
> Cc: Lorenzo Pieralisi <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  drivers/of/address.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/of/address.c b/drivers/of/address.c
> index 02b2903fe9d2..af29b115b8aa 100644
> --- a/drivers/of/address.c
> +++ b/drivers/of/address.c
> @@ -891,19 +891,47 @@ int of_dma_get_range(struct device_node *np, u64 *dma_addr, u64 *paddr, u64 *siz
>  }
>  EXPORT_SYMBOL_GPL(of_dma_get_range);
>  
> +/*
> + * DMA from some device types is always cache-coherent, and in some unfortunate
> + * cases the "dma-coherent" property is not used.
> + */
> +static const char *of_device_dma_coherent_tbl[] = {
> +	/*
> +	 * Virtio MMIO devices are assumed to be cache-coherent when accessing
> +	 * main memory. Neither QEMU nor kvmtool emit "dma-coherent" properties
> +	 * for their generated virtio MMIO device nodes, and the binding
> +	 * documentation doesn't mention them either. When using the DMA API
> +	 * (e.g. because there is an IOMMU in the system), we must report true
> +	 * here to avoid lockups where writes to the vring via a non-coherent
> +	 * mapping are not made visible to the device emulation.
> +	 */
> +	"virtio,mmio",
> +	NULL,
> +};
> +
>  /**
>   * of_dma_is_coherent - Check if device is coherent
>   * @np:	device node
>   *
>   * It returns true if "dma-coherent" property was found
> - * for this device in DT.
> + * for this device in DT or the device is statically known to be
> + * coherent.
>   */
>  bool of_dma_is_coherent(struct device_node *np)
>  {
>  	struct device_node *node = of_node_get(np);
>  
> +	/*
> +	 * Check for implicit DMA coherence first, since we don't want
> +	 * to inherit this.
> +	 */
> +	if (of_device_compatible_match(np, of_device_dma_coherent_tbl)) {
> +		of_node_put(node);
> +		return true;
> +	}
> +
>  	while (node) {
> -		if (of_property_read_bool(node, "dma-coherent")) {
> +		if (of_property_read_bool(node, "dma-coherent")){
>  			of_node_put(node);
>  			return true;
>  		}
> -- 
> 2.1.4