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

[Christian König]

Am 07.08.22 um 19:56 schrieb Rob Clark:
> On Sun, Aug 7, 2022 at 10:38 AM Christian König
> <ckoenig.leichtzumerken@xxxxxxxxx> 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.