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 16:48:03 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr <olekstysh@...il.com>,
        Stefano Stabellini <sstabellini@...nel.org>
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

On 19.04.22 14:17, Oleksandr wrote:
> 
> 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

xen_is_grants_dma_device, please. Normal Xen PV devices are covered by
grants, too, and I'd like to avoid the confusion arising from this.

>> - 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?

Yes, why not.

Maybe rename xen-virtio.c to grant-dma.c?

I'd keep the XEN_VIRTIO related config option, as this will be the normal use
case. grant-dma.c should be covered by a new hidden config option XEN_GRANT_DMA
selected by XEN_VIRTIO.

CONFIG_XEN_VIRTIO should still guard xen_has_restricted_virtio_memory_access().


Juergen

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