linux-kernel - Re: [PATCH 0/5] udmbuf bug fix and some improvements

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Message-ID: <86d7ca9c-1493-474b-95d6-1e38b5703a67@mailbox.org>
Date: Fri, 2 Aug 2024 17:26:21 +0200
From: Michel Dänzer <michel.daenzer@...lbox.org>
To: "Kasireddy, Vivek" <vivek.kasireddy@...el.com>, Huan Yang
 <link@...o.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
 "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "opensource.kernel@...o.com" <opensource.kernel@...o.com>
Subject: Re: [PATCH 0/5] udmbuf bug fix and some improvements

On 2024-08-01 20:32, Kasireddy, Vivek wrote:
> Hi Huan,
> 
>> This patchset attempts to fix some errors in udmabuf and remove the
>> upin_list structure.
>>
>> Some of this fix just gather the patches which I upload before.
>>
>> Patch1
>> ===
>> Try to remove page fault mmap and direct map it.
>> Due to current udmabuf has already obtained and pinned the folio
>> upon completion of the creation.This means that the physical memory has
>> already been acquired, rather than being accessed dynamically. The
>> current page fault method only saves some page table memory.
>>
>> As a result, the page fault mechanism has lost its purpose as a demanding
>> page. Due to the fact that page fault requires trapping into kernel mode
>> and filling in when accessing the corresponding virtual address in mmap,
>> this means that user mode access to virtual addresses needs to trap into
>> kernel mode.
>>
>> Therefore, when creating a large size udmabuf, this represents a
>> considerable overhead.
> Just want to mention that for the main use-case the udmabuf driver is designed for,
> (sharing Qemu Guest FB with Host for GPU DMA), udmabufs are not created very
> frequently. And, I think providing CPU access via mmap is just a backup, mainly
> intended for debugging purposes.

FYI, Mesa now uses udmabuf for supporting dma-bufs with software rendering.


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer