From: "Michael S. Tsirkin" <mst@redhat.com>
To: Keir Fraser <keirf@google.com>
Cc: Jason Wang <jasowang@redhat.com>,
kernel-team@android.com,
virtualization@lists.linux-foundation.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] virtio: Force DMA restricted devices through DMA API
Date: Tue, 19 Jul 2022 07:56:09 -0400 [thread overview]
Message-ID: <20220719074941-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20220719100256.419780-1-keirf@google.com>
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.
> ---
> 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
next prev parent reply other threads:[~2022-07-19 11:56 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-19 10:02 [PATCH] virtio: Force DMA restricted devices through DMA API Keir Fraser
2022-07-19 11:56 ` Michael S. Tsirkin [this message]
2022-07-19 14:05 ` Keir Fraser
2022-07-19 21:31 ` Michael S. Tsirkin
2022-07-19 15:23 ` Christoph Hellwig
2022-07-19 15:46 ` Keir Fraser
2022-07-19 15:51 ` Christoph Hellwig
2022-07-19 16:11 ` Keir Fraser
2022-07-20 5:16 ` Christoph Hellwig
2022-07-20 6:59 ` Michael S. Tsirkin
2022-07-20 8:27 ` Keir Fraser
2022-07-20 9:58 ` Michael S. Tsirkin
2022-07-21 7:37 ` Keir Fraser
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