x86/xsave: fix migration from xsave-capable to xsave-incapable host

x86/xsave: fix migration from xsave-capable to xsave-incapable host

With CPUID features suitably masked this is supposed to work, but was
completely broken (i.e. the case wasn't even considered when the
original xsave save/restore code was written).

First of all, xsave_enabled() wrongly returned the value of
cpu_has_xsave, i.e. not even taking into consideration attributes of
the vCPU in question. Instead this function ought to check whether the
guest ever enabled xsave support (by writing a [non-zero] value to
XCR0). As a result of this, a vCPU's xcr0 and xcr0_accum must no longer
be initialized to XSTATE_FP_SSE (since that's a valid value a guest
could write to XCR0), and the xsave/xrstor as well as the context
switch code need to suitably account for this (by always enforcing at
least this part of the state to be saved/loaded).

This involves undoing large parts of c/s 22945:13a7d1f7f62c ("x86: add
strictly sanity check for XSAVE/XRSTOR") - we need to cleanly
distinguish between hardware capabilities and vCPU used features.

Next both HVM and PV save code needed tweaking to not always save the
full state supported by the underlying hardware, but just the parts
that the guest actually used. Similarly the restore code should bail
not just on state being restored that the hardware cannot handle, but
also on inconsistent save state (inconsistent XCR0 settings or size of
saved state not in line with XCR0).

And finally the PV extended context get/set code needs to use slightly
different logic than the HVM one, as here we can't just key off of
xsave_enabled() (i.e. avoid doing anything if a guest doesn't use
xsave) because the tools use this function to determine host
capabilities as well as read/write vCPU state. The set operation in
particular needs to be capable of cleanly dealing with input that
consists of only the xcr0 and xcr0_accum values (if they're both zero
then no further data is required).

While for things to work correctly both sides (saving _and_ restoring
host) need to run with the fixed code, afaict no breakage should occur
if either side isn't up to date (other than the breakage that this
patch attempts to fix).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Yang Zhang <yang.z.zhang@intel.com>
Acked-by: Keir Fraser <keir@xen.org>