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[Xen-devel] Xen's use of PAT and PV guests

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: [Xen-devel] Xen's use of PAT and PV guests
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 29 Mar 2010 17:35:57 -0700
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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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...



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