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

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Date:   Tue, 19 Apr 2022 15:17:25 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Julien Grall <julien@....org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio
 devices in Xen guests


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