KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES

KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES

Allow FP and SSE state to be saved and restored via KVM_{G,SET}_XSAVE on
XSAVE-capable hosts even if their bits are not exposed to the guest via
XCR0.

Failing to allow FP+SSE first showed up as a QEMU live migration failure,
where migrating a VM from a pre-XSAVE host, e.g. Nehalem, to an XSAVE
host failed due to KVM rejecting KVM_SET_XSAVE.  However, the bug also
causes problems even when migrating between XSAVE-capable hosts as
KVM_GET_SAVE won't set any bits in user_xfeatures if XSAVE isn't exposed
to the guest, i.e. KVM will fail to actually migrate FP+SSE.

Because KVM_{G,S}ET_XSAVE are designed to allowing migrating between
hosts with and without XSAVE, KVM_GET_XSAVE on a non-XSAVE (by way of
fpu_copy_guest_fpstate_to_uabi()) always sets the FP+SSE bits in the
header so that KVM_SET_XSAVE will work even if the new host supports
XSAVE.

Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
Cc: stable@vger.kernel.org
Cc: Leonardo Bras <leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
[sean: add comment, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220824033057.3576315-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>