Messages in this thread | | | Date | Tue, 19 Jul 2022 14:05:58 +0000 | From | Keir Fraser <> | Subject | Re: [PATCH] virtio: Force DMA restricted devices through DMA API |
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On Tue, Jul 19, 2022 at 07:56:09AM -0400, Michael S. Tsirkin wrote: > On Tue, Jul 19, 2022 at 10:02:56AM +0000, Keir Fraser wrote: > > If virtio devices are tagged for "restricted-dma-pool", then that > > pool should be used for virtio ring setup, via the DMA API. > > > > In particular, this fixes virtio_balloon for ARM PKVM, where the usual > > workaround of setting VIRTIO_F_ACCESS_PLATFORM in the virtio device > > doesn't work because the virtio_balloon driver clears the flag. This > > seems a more robust fix than fiddling the flag again. > > > > Signed-off-by: Keir Fraser <keirf@google.com> > > > So the reason balloon disables ACCESS_PLATFORM is simply > because it passes physical addresses to device and > expects device to be able to poke at them. > > I worry about modifying DMA semantics yet again - it has as much of a > chance to break some legacy configs as it has to fix some. > > > And I don't really know much about restricted-dma-pool but > I'd like to understand why does it make sense to set it for > the balloon since it pokes at all and any system memory.
So this is set in the device tree by the host, telling it to bounce all DMA through a restricted memory window (basically swiotlb). The original reason is simply to isolate DMA, to the extent possible, on IOMMU-less systems.
However it is also useful for PKVM because the host is not trusted to access ordinary protected VM memory. To allow I/O via the host, restricted-dma-pool is used to cause a bounce aperture to be allocated during VM boot, which is then explicitly shared with the host. For correct PKVM virtio operation, all data *and metadata* (virtio rings and descriptors) must be allocated in or bounced through this aperture.
Insofar as virtio device accesses to virtio rings in guest memory essentially *are* DMA (from the pov of the guest), I think it makes sense to respect the bounce buffer for those rings, if so configured by the device tree.
> > --- > > drivers/virtio/virtio_ring.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > index a5ec724c01d8..12be2607c648 100644 > > --- a/drivers/virtio/virtio_ring.c > > +++ b/drivers/virtio/virtio_ring.c > > @@ -12,6 +12,7 @@ > > #include <linux/hrtimer.h> > > #include <linux/dma-mapping.h> > > #include <linux/spinlock.h> > > +#include <linux/swiotlb.h> > > #include <xen/xen.h> > > > > #ifdef DEBUG > > @@ -248,6 +249,13 @@ static bool vring_use_dma_api(struct virtio_device *vdev) > > if (!virtio_has_dma_quirk(vdev)) > > return true; > > > > + /* If the device is configured to use a DMA restricted pool, > > + * we had better use it. > > + */ > > + if (IS_ENABLED(CONFIG_DMA_RESTRICTED_POOL) && > > + is_swiotlb_for_alloc(vdev->dev.parent)) > > + return true; > > + > > /* Otherwise, we are left to guess. */ > > /* > > * In theory, it's possible to have a buggy QEMU-supposed > > -- > > 2.37.0.170.g444d1eabd0-goog > >
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