Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio devices in Xen guests

From: Oleksandr <olekstysh@gmail.com>
To: Stefano Stabellini <sstabellini@kernel.org>,
	Juergen Gross <jgross@suse.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Julien Grall <julien@xen.org>,
	"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio devices in Xen guests
Date: Tue, 19 Apr 2022 15:17:25 +0300	[thread overview]
Message-ID: <6a04cc34-fbb3-44d8-c1a4-03bda5b3deb1@gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2204181202080.915916@ubuntu-linux-20-04-desktop>

Hello Stefano, Juergen

On 18.04.22 22:11, Stefano Stabellini wrote:
> On Mon, 18 Apr 2022, Oleksandr wrote:
>> On 16.04.22 09:07, Christoph Hellwig wrote:
>>
>> Hello Christoph
>>
>>> On Fri, Apr 15, 2022 at 03:02:45PM -0700, Stefano Stabellini wrote:
>>>> This makes sense overall. Considering that the swiotlb-xen case and the
>>>> virtio case are mutually exclusive, I would write it like this:
>>> Curious question:  Why can't the same grant scheme also be used for
>>> non-virtio devices?  I really hate having virtio hooks in the arch
>>> dma code.  Why can't Xen just say in DT/ACPI that grants can be used
>>> for a given device?
> [...]
>
>> This patch series tries to make things work with "virtio" devices in Xen
>> system without introducing any modifications to code under drivers/virtio.
>
> Actually, I think Christoph has a point.
>
> There is nothing inherently virtio specific in this patch series or in
> the "xen,dev-domid" device tree binding.

Although the main intention of this series was to enable using virtio 
devices in Xen guests, I agree that nothing in new DMA ops layer 
(xen-virtio.c) is virtio specific (at least at the moment). Regarding 
the whole patch series I am not quite sure, as it uses 
arch_has_restricted_virtio_memory_access().

>   Assuming a given device is
> emulated by a Xen backend, it could be used with grants as well.
>
> For instance, we could provide an emulated e1000 NIC with a
> "xen,dev-domid" property in device tree. Linux could use grants with it
> and the backend could map the grants. It would work the same way as
> virtio-net/block/etc. Passthrough devices wouldn't have the
> "xen,dev-domid" property, so no problems.
>
> So I think we could easily generalize this work and expand it to any
> device. We just need to hook on the "xen,dev-domid" device tree
> property.
>
> I think it is just a matter of:
> - remove the "virtio,mmio" check from xen_is_virtio_device
> - rename xen_is_virtio_device to something more generic, like
>    xen_is_grants_device
> - rename xen_virtio_setup_dma_ops to something more generic, like
>    xen_grants_setup_dma_ops
>
> And that's pretty much it.

+ likely renaming everything in that patch series not to mention virtio 
(mostly related to xen-virtio.c internals).

Stefano, thank you for clarifying Christoph's point.

Well, I am not against going this direction. Could we please make a 
decision on this? @Juergen, what is your opinion?

-- 
Regards,

Oleksandr Tyshchenko

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