Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly

[Paolo Bonzini]

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