linux-kernel - Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

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Date:   Mon, 23 Apr 2018 18:41:41 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        Wei Liu <wei.liu2@...rix.com>
Cc:     Roger Pau Monné <roger.pau@...rix.com>,
        jgross@...e.com, Artem Mygaiev <Artem_Mygaiev@...m.com>,
        Dongwon Kim <dongwon.kim@...el.com>, konrad.wilk@...cle.com,
        airlied@...ux.ie,
        "Oleksandr_Andrushchenko@...m.com" <Oleksandr_Andrushchenko@...m.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "Potrola, MateuszX" <mateuszx.potrola@...el.com>,
        xen-devel@...ts.xenproject.org, daniel.vetter@...el.com
Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper
 DRM driver

On 04/23/2018 08:10 AM, Oleksandr Andrushchenko wrote:
> On 04/23/2018 02:52 PM, Wei Liu wrote:
>> On Fri, Apr 20, 2018 at 02:25:20PM +0300, Oleksandr Andrushchenko wrote:
>>>>>      the gntdev.
>>>>>
>>>>> I think this is generic enough that it could be implemented by a
>>>>> device not tied to Xen. AFAICT the hyper_dma guys also wanted
>>>>> something similar to this.
>>>> You can't just wrap random userspace memory into a dma-buf. We've
>>>> just had
>>>> this discussion with kvm/qemu folks, who proposed just that, and
>>>> after a
>>>> bit of discussion they'll now try to have a driver which just wraps a
>>>> memfd into a dma-buf.
>>> So, we have to decide either we introduce a new driver
>>> (say, under drivers/xen/xen-dma-buf) or extend the existing
>>> gntdev/balloon to support dma-buf use-cases.
>>>
>>> Can anybody from Xen community express their preference here?
>>>
>> Oleksandr talked to me on IRC about this, he said a few IOCTLs need to
>> be added to either existing drivers or a new driver.
>>
>> I went through this thread twice and skimmed through the relevant
>> documents, but I couldn't see any obvious pros and cons for either
>> approach. So I don't really have an opinion on this.
>>
>> But, assuming if implemented in existing drivers, those IOCTLs need to
>> be added to different drivers, which means userspace program needs to
>> write more code and get more handles, it would be slightly better to
>> implement a new driver from that perspective.
> If gntdev/balloon extension is still considered:
>
> All the IOCTLs will be in gntdev driver (in current xen-zcopy
> terminology):
>  - DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS
>  - DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
>  - DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE
>
> Balloon driver extension, which is needed for contiguous/DMA
> buffers, will be to provide new *kernel API*, no UAPI is needed.
>


So I am obviously a bit late to this thread, but why do you need to add
new ioctls to gntdev and balloon? Doesn't this driver manage to do what
you want without any extensions?

-boris