[PATCH v1] x86/fpu: Fix guest FPU state buffer allocation size

From: Stanislav Spassov
Date: Tue Feb 18 2025 - 09:12:18 EST


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@xxxxxxxxx>
---
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 1209c7aebb21..36df548acc40 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;
--
2.47.1