Messages in this thread | | | Date | Sun, 12 Dec 2021 10:10:19 +0100 | Subject | Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly | From | Paolo Bonzini <> |
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On 12/12/21 02:50, Tian, Kevin wrote: >> >> If there are other XFD controlled facilities in the future, then it will >> be NR_USED_XFD_CONTROLLED_FACILITIES * 2 VMEXITs per thread which >> uses >> them. Not the end of the world either. >> >> Looking at the targeted application space it's pretty unlikely that >> tasks which utilize AMX are going to be so short lived that the overhead >> of these VMEXITs really matters. >> >> This of course can be revisited when there is a sane use case, but >> optimizing for it prematurely does not buy us anything else than >> pointless complexity. > It may affect guest which still uses CR0.TS to do lazy save. But likely > modern OSes all move to eager save approach so always trapping #NM > should be fine.
You also don't need to trap #NM if CPUID includes no dynamic bits, because then XFD will never be nonzero.
Paolo
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