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Subject: Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper
 DRM driver
To:     Wei Liu <wei.liu2@citrix.com>, jgross@suse.com,
        Artem Mygaiev <Artem_Mygaiev@epam.com>,
        Dongwon Kim <dongwon.kim@intel.com>, konrad.wilk@oracle.com,
        airlied@linux.ie, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org,
        "Potrola, MateuszX" <mateuszx.potrola@intel.com>,
        daniel.vetter@intel.com, xen-devel@lists.xenproject.org,
        boris.ostrovsky@oracle.com,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
References: <20180417075928.GT31310@phenom.ffwll.local>
 <20180417205744.GA15930@downor-Z87X-UD5H>
 <41487acb-a67a-8933-d0c3-702c19b0938e@gmail.com>
 <20180418073508.ptvntwedczpvl7bx@MacBook-Pro-de-Roger.local>
 <ebcbf6ce-1d7a-5f3e-cff8-546d1c010e59@gmail.com>
 <20180418101058.hyqk3gr3b2ibxswu@MacBook-Pro-de-Roger.local>
 <20180420071914.GG31310@phenom.ffwll.local>
 <76cdc65a-7bb1-9377-7bc5-6164e32f7b5d@gmail.com>
 <20180423115242.ywdwqblj2aseu3fr@citrix.com>
 <61105351-8896-072b-abf0-757c7f6c0edf@gmail.com>
 <20180424115437.GT31310@phenom.ffwll.local>
Cc:     "Oleksandr_Andrushchenko@epam.com" <Oleksandr_Andrushchenko@epam.com>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <18ab5f76-00b0-42a0-fcb8-e0cbf4cdd527@gmail.com>
Date:   Tue, 24 Apr 2018 14:59:39 +0300
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On 04/24/2018 02:54 PM, Daniel Vetter wrote:
> On Mon, Apr 23, 2018 at 03:10:35PM +0300, 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):
I was lazy to change dumb to dma-buf, so put this notice ;)
>>   - DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS
>>   - DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
>>   - DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE
> s/DUMB/DMA_BUF/ please. This is generic dma-buf, it has nothing to do with
> the dumb scanout buffer support in the drm/gfx subsystem. This here can be
> used for any zcopy sharing among guests (as long as your endpoints
> understands dma-buf, which most relevant drivers do).
Of course, please see above
> -Daniel
>
>> Balloon driver extension, which is needed for contiguous/DMA
>> buffers, will be to provide new *kernel API*, no UAPI is needed.
>>
>>> Wei.
>> Thank you,
>> Oleksandr
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