[Bug 90037] [xen iommu] After upgrading to Linux 3.19, desktop no longer works in Xen 4.5.0 dom0
bugzilla-daemon at freedesktop.org
bugzilla-daemon at freedesktop.org
Tue Aug 18 07:52:13 PDT 2015
https://bugs.freedesktop.org/show_bug.cgi?id=90037
--- Comment #9 from David Woodhouse <dwmw2 at infradead.org> ---
It's odd that it was triggered (in the Xen case) by a PAT patch.
What was the actual effect of that patch on the caching mode used by the
machine in question?
> [ +0.005382] dmar: IOMMU 1: reg_base_addr fed91000 ver 1:0 cap
c0000020230272 ecap 1000
cap & (1<<4) is set, which is the RWBF bit:
1: Indicates software must explicitly flush
the write buffers to ensure updates made to
memory-resident remapping structures are
visible to hardware.
ecap & (1<<0) is clear, which is the Coherency bit:
This field indicates if hardware access to the
root, context, extended-context and
interrupt-remap tables, and second-level
paging structures for requests-without-
PASID, are coherent (snooped) or not.
• 0:Indicates hardware accesses to
remapping structures are non-coherent.
So basically this hardware is in a mode where the IOMMU page tables are
non-cache coherent. Not only do you have to clflush every cache line in the
page tables to main memory when you write it, but you *also* have to jump
through hoops to ensure that the writes are pushed through chipset-specific
write buffers (see §6.8 of the VT-d specification).
That may help to explain why a seemingly innocent PAT change might have
triggered something odd. But it would be good to know precisely what went
wrong.
Also, does it help to add 'iommu=pt' to the kernel command line? That would
make the IOMMU use a 1:1 mapping of all memory, rather than dynamically setting
up mappings.
You say it can be reproduced without Xen, with Linux >= 3.7 — can you show the
details of that please? And if it doesn't occur in 3.6, can you also bisect the
non-Xen case to find when it started happening, please?
Thanks,
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