linux-kernel - Re: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

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Message-ID: <2d9339db-4d26-4ff7-8390-5ac5c0a92401@amd.com>
Date: Fri, 30 Jan 2026 09:46:55 +0100
From: Christian König <christian.koenig@....com>
To: Jason Gunthorpe <jgg@...pe.ca>, "Tian, Kevin" <kevin.tian@...el.com>
Cc: Leon Romanovsky <leon@...nel.org>, Pranjal Shrivastava
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Subject: Re: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

On 1/29/26 15:58, Jason Gunthorpe wrote:
> On Thu, Jan 29, 2026 at 07:06:37AM +0000, Tian, Kevin wrote:
>> Bear me if it's an ignorant question.
>>
>> The commit msg of patch6 says that VFIO doesn't tolerate unbounded
>> wait, which is the reason behind the 2nd timeout wait here.
> 
> As far as I understand dmabuf design a fence wait should complete
> eventually under kernel control, because these sleeps are
> sprinkled all around the kernel today.

Well it's a bit different, but we indeed guarantee that dma_fences complete in finite time.

> I suspect that is not actually true for every HW, probably something
> like "shader programs can run forever technically".

Nope, stuff like that is strictly forbidden.

Regards,
Christian.

> 
> We can argue if those cases should not report revocable either, but at
> least this will work "correctly" even if it takes a huge amount of
> time.
> 
> I wouldn't mind seeing a shorter timeout and print on the fence too
> just in case.
> 
> Jason