I'm looking again at what it will take to reconcile Xen's PAT setup with
the standard Linux one so that we can enable PAT use in pvops kernels.
Just for reference, this is the Linux vs Xen vs default PAT setups:
Index PTE flags Linux Xen Default
0 WB WB WB
1 PWT WC WT WT
2 PCD UC- UC- UC-
3 PCD PWT UC UC UC
4 PAT WB WC WB
5 PAT PWT WC WP WT
6 PAT PCD UC- UC UC-
7 PAT PCD PWT UC UC UC
Originally I was thinking of a moderately complex scheme in which an ELF
node on the dom0 kernel could determine the system-wide Xen PAT MSR, and
then the kernel ELF notes on subsequent domains would determine whether
the PAT CPU feature flag is enabled or not.
However this has several problems:
1. it is fairly complex
2. if dom0 sets the PAT configuration to something strange, it may
completely break other PV guests entirely (since it might
effectively change the meaning of PCD+PWT globally)
3. disabling the PAT CPU feature flag is meaningless, as its only
effect is to say "there's no PAT, so PCD/PWT have their default
behaviours", which is definitely not true in general
Linux only uses the first 4 PAT entries, and repeats it, effectively
making the PAT pte flag a don't-care. In those 4 entries, the Linux,
Xen and Default configurations are identical aside from Linux using WC
rather than WT.
It therefore seems to me that if I make Linux:
1. never set the PAT flag (which it won't anyway),
2. check that the value written to IA32_PAT is as expected, but
otherwise ignore it, and
3. use WT rather than WC
then it all should just work. I'm not completely confident in the third
point though, since I'm not quite sure about the full set of differences
between WT and WC, and their respective interactions with the MTRR, and
whether that would break anything. At first glance it seems pretty safe
though...
Thoughts?
J
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