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 <will.deacon@xxxxxxx>
> 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 <lorenzo.pieralisi@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
> 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
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