linux-kernel - Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Add ioctl to query mmap info

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Date:   Sun, 7 Aug 2022 20:05:15 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Rob Clark <robdclark@...il.com>
Cc:     Christian König <christian.koenig@....com>,
        dri-devel@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
        freedreno@...ts.freedesktop.org,
        Rob Clark <robdclark@...omium.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Jérôme Pouiller <jerome.pouiller@...abs.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Add ioctl to query mmap info

Am 07.08.22 um 19:56 schrieb Rob Clark:
> On Sun, Aug 7, 2022 at 10:38 AM Christian König
> <ckoenig.leichtzumerken@...il.com> wrote:
>> [SNIP]
>> And exactly that was declared completely illegal the last time it came
>> up on the mailing list.
>>
>> Daniel implemented a whole bunch of patches into the DMA-buf layer to
>> make it impossible for KVM to do this.
> This issue isn't really with KVM, it is not making any CPU mappings
> itself.  KVM is just making the pages available to the guest.

Well I can only repeat myself: This is strictly illegal.

Please try this approach with CONFIG_DMABUF_DEBUG set. I'm pretty sure 
you will immediately run into a crash.

See this here as well 
https://elixir.bootlin.com/linux/v5.19/source/drivers/dma-buf/dma-buf.c#L653

Daniel intentionally added code to mangle the page pointers to make it 
impossible for KVM to do this.

If the virtio/virtgpu UAPI was build around the idea that this is 
possible then it is most likely fundamental broken.

Regards,
Christian.