[Bug 107561] 4.2 breaks PCI passthrough in QEMU/KVM

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=107561

schefister@gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |schefister@gmail.com

--- Comment #4 from schefister@gmail.com ---
I narrowed it down earlier to "simplify kvm_mtrr_get_guest_memory_type".
Actually this was the commit to break it, but it is only a small part of the
feauture set implementing vMTRR. To my knowledge 4.1.x versions didn't have
this code set at all. It was introduced in 4.2.
What kvm_mtrr_get_guest_memory_type() does is basically decides what cache type
to use (Uncachable, Write-Back, or Write-Through) based on the function
mtrr_for_each_mem_type(). This kvm_mtrr_get_guest_memory_type function is only
called by vmx_get_mt_mask. This is all related to how the hardware is set up
regarding IOMMU, MMIO, VT-d and EPT andress translation.

For experimenting purposes I simply modified kvm_mtrr_get_guest_memory_type to
always return MTRR_TYPE_WRBACK and that fixed the issue. Guest boots correctly. 
Although I know, that hardcoding Write-back mode is not a viable long-term
solution. It seems as the new function (in 4.2 and onward) does not return the
proper cache type in case of pci passthrough configurations. 

I would also like to point out, that
https://bugzilla.kernel.org/show_bug.cgi?id=107921 may be related. The other
bug mentions very similar hardware to mine, and issues started the same time
(upgrading to 4.2.x) with a pci passthrough configuration.

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