linux-kernel - [tip: x86/fpu] x86/fpu: Initialize guest FPU permissions from guest defaults

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Message-ID: <175080785981.406.5093475254017480218.tip-bot2@tip-bot2>
Date: Tue, 24 Jun 2025 23:30:59 -0000
From: "tip-bot2 for Chao Gao" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Chang S. Bae" <chang.seok.bae@...el.com>, Chao Gao <chao.gao@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, John Allen <john.allen@....com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject:
 [tip: x86/fpu] x86/fpu: Initialize guest FPU permissions from guest defaults

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     7c2c89364d9219f3a31a9a930476de5c154f13bd
Gitweb:        https://git.kernel.org/tip/7c2c89364d9219f3a31a9a930476de5c154f13bd
Author:        Chao Gao <chao.gao@...el.com>
AuthorDate:    Thu, 22 May 2025 08:10:05 -07:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Tue, 24 Jun 2025 13:46:32 -07:00

x86/fpu: Initialize guest FPU permissions from guest defaults

Currently, fpu->guest_perm is copied from fpu->perm, which is derived from
fpu_kernel_cfg.default_features.

Guest defaults were introduced to differentiate the features and sizes of
host and guest FPUs. Copying guest FPU permissions from the host will lead
to inconsistencies between the guest default features and permissions.

Initialize guest FPU permissions from guest defaults instead of host
defaults. This ensures that any changes to guest default features are
automatically reflected in guest permissions, which in turn guarantees
that fpstate_realloc() allocates a correctly sized XSAVE buffer for guest
FPUs.

Suggested-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Reviewed-by: John Allen <john.allen@....com>
Link: https://lore.kernel.org/all/20250522151031.426788-3-chao.gao%40intel.com
---
 arch/x86/kernel/fpu/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index aa72523..94706a5 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -562,8 +562,14 @@ void fpstate_reset(struct fpu *fpu)
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
 	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
 	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
-	/* Same defaults for guests */
-	fpu->guest_perm = fpu->perm;
+
+	fpu->guest_perm.__state_perm	= guest_default_cfg.features;
+	fpu->guest_perm.__state_size	= guest_default_cfg.size;
+	/*
+	 * User features and sizes are always identical between host and
+	 * guest FPUs, which allows for common guest and userspace ABI.
+	 */
+	fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
 }

 static inline void fpu_inherit_perms(struct fpu *dst_fpu)