linux-kernel - [tip: x86/fpu] x86/fpu: Fix guest FPU state buffer allocation size

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Message-ID: <174014620007.10177.13346155405991854999.tip-bot2@tip-bot2>
Date: Fri, 21 Feb 2025 13:56:40 -0000
From: "tip-bot2 for Stanislav Spassov" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Stanislav Spassov <stanspas@...zon.de>, Ingo Molnar <mingo@...nel.org>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/fpu] x86/fpu: Fix guest FPU state buffer allocation size

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

Commit-ID:     1937e18cc3cf27e2b3ef70e8c161437051ab7608
Gitweb:        https://git.kernel.org/tip/1937e18cc3cf27e2b3ef70e8c161437051ab7608
Author:        Stanislav Spassov <stanspas@...zon.de>
AuthorDate:    Tue, 18 Feb 2025 14:10:45 
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 21 Feb 2025 14:47:26 +01:00

x86/fpu: Fix guest FPU state buffer allocation size

Ongoing work on an optimization to batch-preallocate vCPU state buffers
for KVM revealed a mismatch between the allocation sizes used in
fpu_alloc_guest_fpstate() and fpstate_realloc(). While the former
allocates a buffer sized to fit the default set of XSAVE features
in UABI form (as per fpu_user_cfg), the latter uses its ksize argument
derived (for the requested set of features) in the same way as the sizes
found in fpu_kernel_cfg, i.e. using the compacted in-kernel
representation.

The correct size to use for guest FPU state should indeed be the
kernel one as seen in fpstate_realloc(). The original issue likely
went unnoticed through a combination of UABI size typically being
larger than or equal to kernel size, and/or both amounting to the
same number of allocated 4K pages.

Fixes: 69f6ed1d14c6 ("x86/fpu: Provide infrastructure for KVM FPU cleanup")
Signed-off-by: Stanislav Spassov <stanspas@...zon.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20250218141045.85201-1-stanspas@amazon.de
---
 arch/x86/kernel/fpu/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1209c7a..36df548 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -220,7 +220,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
 	struct fpstate *fpstate;
 	unsigned int size;

-	size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
+	size = fpu_kernel_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
 	fpstate = vzalloc(size);
 	if (!fpstate)
 		return false;