[Pkg-xen-devel] Bug#988333: Bug#988333: libxenmisc4.16: libxl fails to grant necessary I/O memory access for gfx_passthru of Intel IGD

[Marek Marczykowski-Górecki]

On Mon, Mar 07, 2022 at 10:11:29AM -0500, Chuck Zmudzinski wrote:
> Detailed description of how I discovered the patch that fixes the bug and
> enables Intel IGD passthrough to Bullseye when using the traditional Qemu
> device model:
> 
> A long time ago, during the development of Xen 4.5 in 2014, two patches
> implemented a change to the way permission is granted for an unprivileged
> domain to access PCI/VGA-related I/O memory. Prior to this, AFAICT,
> permission was implicitly granted to access the memory the domain requested
> when a PCI device being passed to the domain was being configured. After the
> change, permission to access such memory is not granted without prior
> explicit permission being configured, and this is still the current
> behavior.
> 
> The relevant patches are:
> 
> 1. https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=abfb006f1ff4af1424e5b0d0589f0226377fda36
> 
> and
> 
> 2. https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=0561e1f01e87b777bcc47971e4ae1f420314f4a0
> 
> The first of these patches intended to implement explicit granting of
> permission to access the I/O memory that is needed to support the
> gfx_passthru feature in libxl_pci.c, in the
> libxl__grant_vga_iomem_permission function. The second patch implements the
> removal of implicit permission to access the PCI/VGA-related I/O memory and
> causes requests to access such memory by a domain to be denied unless prior
> explicit permission has been configured.
> 
> Specifically, the first patch adds 32 (0x20) units (I presume bytes) of data

That's in pages.

> starting at memory address 0xa0000 >> XC_PAGE_SHIFT to the memory the domain
> is permitted to access. XC_PAGE_SHIFT is 12, so this memory range shows up
> in the logs when running Xen 4.5 as:
> 
> memory_map:add: dom1 gfn=a0 mfn=a0 nr=20
> 
> But my testing of these old patches with added custom logging shows that
> another two units (bytes?) are needed:
> 
> memory_map:access not permitted: dom1 gfn=fdffc mfn=cc490 nr=2

You may be interested in this patch:
https://github.com/QubesOS/qubes-vmm-xen/blob/xen-4.14/patch-fix-igd-passthrough-with-linux-stubdomain.patch

Qubes OS uses qemu-upstream in a Linux-based stubdomain, and with the
above patch applied, IGD passthrough works, at least to a Linux HVM.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
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