CP-308875: set Xen PCI MMIO BAR to WB #6591
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robhoes
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I'll rebase this on #6566 (comment) once that is in and change the defaults there. |
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Rebased on the other PR by tweaking the newly introduced xenopsd.conf default, thanks for pointing it out. |
psafont
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last-genius
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The default for the Xen PCI MMIO BAR is UnCachable. Setting this to WriteBack in the MTRR shows a massive performance improvement on AMD, at least for Linux guests. On Intel this is already set to WriteBack by Xen via another mechanism. To be effective this also requires the corresponding [Xen commit](https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=22650d6054625be10172fe0c78b9cadd1a39bd63), old versions will just ignore this xenstore key. The optimization is not enabled by default in Xen due to the wide range of guests it supports, but XAPI supports a much narrower set of guest OSes. Setting the cache attribute to WB is done by setting UC=false. Signed-off-by: Edwin Török <[email protected]>
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andyhhp
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| This is useful for AMD, and mostly a noop on Intel (which achieves a similar | ||
| effect using Intel-only features in Xen) |
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The part in brackets is going to go stale quickly. In order to support encrypted memory (and later, encrypted VMs), we're going to have to stop using iPAT on Intel, at which point Intel will become like AMD and this control will be relevant.
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ok, so doing this uniformly now on both Intel and AMD actually future proofs us against that. You can then change the Intel side from iPAT in Xen without further toolstack changes?
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The default for the Xen PCI MMIO BAR is UnCachable. Setting this to WriteBack in the MTRR shows a massive performance improvement on AMD, at least for Linux guests.
On Intel this is already set to WriteBack by Xen via another mechanism.
To be effective this also requires the corresponding Xen commit, old versions will just ignore this xenstore key.
The optimization is not enabled by default in Xen due to the wide range of guests it supports, but XAPI supports a much narrower set of guest OSes.
Setting the cache attribute to WB is done by setting UC=0.