Re: [PATCH v4 10/10] x86/xsaves: Re-enable XSAVES

[Dave Hansen]

It's my fault, but you also need to go update
	
	fpu__xfeature_set_state()
and
	__raw_xsave_addr()

The theoretical problem is that you might ask for a __raw_xsave_addr()
of a component which has been compacted out of an XSAVES buffer and thus
has no address.  We could work around this by doing a memmove() and
moving the components "up" after the one we are trying to set in order
to make space.

But, since we *always* call XSAVES with an instruction mask of -1 and
end up with a requested feature bitmap (RFBM) equal to XCR0, I think we
can do a shortcut because we'll practically *always* have an
xcomp_bv==RFBM==XCR0, which means that all (present) components will
always have an address.

So, the alternative to doing the memmove() is to add some WARN_ON_FPU()
checks to enforce xcomp_bv==RFBM==XCR0 in places where we call
XSAVES/XRSTORS and __raw_xsave_addr(), maybe more.