Re: [PATCH] virtio: Force DMA restricted devices through DMA API

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Keir Fraser <keirf@google.com>
Cc: kernel-team@android.com, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.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

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