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

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Message-Id: <20230914032334.75212-7-weijiang.yang@intel.com>
Date:   Wed, 13 Sep 2023 23:23:32 -0400
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     dave.hansen@...el.com, tglx@...utronix.de, peterz@...radead.org,
        seanjc@...gle.com, pbonzini@...hat.com, rick.p.edgecombe@...el.com,
        kvm@...r.kernel.org, yang.zhong@...el.com, jing2.liu@...el.com,
        chao.gao@...el.com, Yang Weijiang <weijiang.yang@...el.com>
Subject: [RFC PATCH 6/8] x86/fpu/xstate: Opt-in kernel dynamic bits when calculate guest xstate size

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@...el.com>
---
 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