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Re: Linux pin_user_pages_fast fails on Xen
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Stefano Stabellini <stefano.stabellini@xxxxxxx>
- Date: Fri, 9 Sep 2022 13:25:35 -0700
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- Cc: Stefano Stabellini <stefano.stabellini@xxxxxxx>, <boris.ostrovsky@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>, <JESHWANTHKUMAR.NK@xxxxxxx>, <Devaraj.Rangasamy@xxxxxxx>, <Mythri.Pandeshwarakrishna@xxxxxxx>, <SivaSangeetha.SK@xxxxxxx>, <Rijo-john.Thomas@xxxxxxx>
- Delivery-date: Fri, 09 Sep 2022 20:26:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri, 9 Sep 2022, Juergen Gross wrote:
> On 09.09.22 04:11, Stefano Stabellini wrote:
> > Adding more people in CC
> >
> > On Thu, 8 Sep 2022, Stefano Stabellini wrote:
> > > Hi Juergen,
> > >
> > > A colleague is seeing a failure on x86 in Linux Dom0. The failure is
> > > pin_user_pages_fast with addresses that correspond to foreign memory
> > > pages:
> > >
> > > - QEMU maps a domU address using dma_memory_map (xen_map_cache)
> > > - QEMU calls an IOCTL to the TEE subsystem with the Virtual Address
> > > returned by dma_memory_map
> > > - Linux tee_shm_register->pin_user_pages_fast Returns -14 -
> > > drivers/tee/tee_shm.c
> > >
> > > Once upon a time it used to be the case that get_user_pages_fast would
> > > fail on Xen because we didn't have a struct page corresponding to
> > > foreign memory mappings. But that hasn't been the case for years now.
> > >
> > > Any other ideas why it would fail?
>
> I think we can expect that access_ok() isn't failing.
>
> I assume the mapping was done allowing writes (sorry for paranoia mode)?
I was told it was verified: QEMU could read and write to the VA returned
by dma_memory_map. From /proc/<qemu-pid>/maps, the VA assigned after the
mapping is pointing to /dev/xen/privcmd.
> Other than that I'm not having enough memory management skills. It might be
> related to mmap()-ed foreign pages having _PAGE_SPECIAL set, though.
Do we still set PAGE_SPECIAL for foreign mapped pages? It looks like it
is not there anymore? If PAGE_SPECIAL is not there, then they really
should look like regular pages?
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