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Re: [Qemu-devel] [PATCH v2] Add udmabuf misc device

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH v2] Add udmabuf misc device
Date: Fri, 6 Apr 2018 11:33:07 +0200
User-agent: NeoMutt/20180323


> The pages backing a DMA-buf are not allowed to move (at least not without a
> patch set I'm currently working on), but for certain MM operations to work
> correctly you must be able to modify the page tables entries and move the
> pages backing them around.
> For example try to use fork() with some copy on write pages with this
> approach. You will find that you have only two options to correctly handle
> this.

The fork() issue should go away with shared memory pages (no cow).
I guess this is the reason why vgem is internally backed by shmem.

Hmm.  So I could try to limit the udmabuf driver to shmem too (i.e.
have the ioctl take a shmem filehandle and offset instead of a virtual

But maybe it is better then to just extend vgem, i.e. add support to
create gem objects from existing shmem.



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