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@amazon.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250218141045.85201-1-stanspas@amazon.de

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1209c7aebb211f10c5e021724da0414b2f21a27f..36df548acc403c8b3d4c16869e940dfcdf596830 100644 (file)
--- 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;