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Re: [Xen-devel][Patch] MTRR virtualization

On 19 Jun 2006, at 09:29, Dong, Eddie wrote:

For example, a guest may set video frame buffer to be "WC". But in Xen
environment, as we are using memory (RAM) to back for guest video frame,

we have to set the memory type to be "WB" otherewise the access using
aliase address in Qemu (WB attribute) may cause mca.
Current shadow simply copy guest PAT entry to shadow PTE even for video
frame buffer, this works as if the guest is setting PAT as "WB" and MTRR
as "WC".
On the otherhand, if guest is setting PAT as "WC", and MTRR as "WB", we
problem. Because in this case, shadow PAT="WC", machine MTRR = "WB",
and thus we get "WC" for this video frame. MCA may happen.

The correct approach right now is to set WB for everything. HVM guests currently never directly map I/O resources. Anything more complicated than that, until we actually support direct I/O access, is quite mad.

 -- Keir

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