Re: [Xen-devel] Guest-vs-Host MTRR/PAT conflict and a crash?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 12/12/07 22:13, "David Stone" <unclestoner@xxxxxxxxx> wrote:

> root@localhost xen]# (XEN) mtrr.c:552:d1 Conflict occurs for a given
> guest l1e flags:63 at 10000000 (the effective mm type:6), because the
> host mtrr type is:0
> (XEN) CPU 1: Machine Check Exception: 0000000000000005
> (XEN) Bank 0: b200004000000800
> (XEN) Bank 5: b200121020080400
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 1:
> (XEN) CPU context corrupt****************************************
> (XEN)
> (XEN) Reboot in five seconds..

That looks like the CPU toasted itself. Bits 0-16 == 0x0400 in a
machine-check status register means 'CPU internal timer error'. Perhaps this
#MC means something else in the context of VT-d though? We probably need
someone from Intel to help decode what happened here.

> I guess my question is can someone give a higher-level explaination of
> how MTRR/PAT is supposed to work between Xen (the real MTRRs and real
> PATs in the real ("shadow") PTEs) and an HVM DomU (virtual MTRRs and
> fake PATs in the fake PTEs)?

You pretty much described it yourself. It tries to pick a shadow-pte PAT
value which, coupled with the physical MTRR value for that physical address,
will give the same cache attribute as the guest virtual PAT/MTRR
combination. If it can't find a match (not entirely unlikely I suspect) then
we print a warning and fall back to UC. This code is quite 'fresh' and
definitely open for changes based on real-world testing.

 -- Keir



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