[PATCH v6 4/7] x86/fpu: Initialize guest FPU permissions from guest defaults

Chao Gao
2025-05-06T09:33:16Z

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@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
v6: Drop vcpu_fpu_config.user_* and collect reviews (Rick)
---
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 2cd5e1910ff8..444e517a8648 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -553,8 +553,14 @@ 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 remain the same between guest FPUs
+ * and host FPUs.
+ */
+ fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
}

static inline void fpu_inherit_perms(struct fpu *dst_fpu)
--
2.47.1