[RFC PATCH 6/8] x86/fpu/xstate: Opt-in kernel dynamic bits when calculate guest xstate size

[Yang Weijiang]

When user space requests guest xstate permits, the sufficient xstate size
is calculated from permitted mask. Currently the max guest permits are set
to fpu_kernel_cfg.default_features, and the latter doesn't include kernel
dynamic xfeatures, so add them back for correct guest fpstate size.

If guest dynamic xfeatures are enabled, KVM re-allocates guest fpstate area
with above resulting size before launches VM.

Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 arch/x86/kernel/fpu/xstate.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 4753c677e2e1..c5d903b4df4d 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1636,9 +1636,17 @@ static int __xstate_request_perm(u64 permitted, u64 requested, bool guest)
 
 	/* Calculate the resulting kernel state size */
 	mask = permitted | requested;
-	/* Take supervisor states into account on the host */
+	/*
+	 * Take supervisor states into account on the host. And add
+	 * kernel dynamic xfeatures to guest since guest kernel may
+	 * enable corresponding CPU feaures and the xstate registers
+	 * need to be saved/restored properly.
+	 */
 	if (!guest)
 		mask |= xfeatures_mask_supervisor();
+	else
+		mask |= fpu_kernel_dynamic_xfeatures;
+
 	ksize = xstate_calculate_size(mask, compacted);
 
 	/* Calculate the resulting user state size */
-- 
2.27.0