[patch V2 50/52] x86/fpu: Dont store PKRU in xstate in fpu_reset_fpstate()

Thomas Gleixner
2021-06-14T16:40:52Z

PKRU for a task is stored in task->thread.pkru when the task is scheduled
out. For 'current' the authoritative source of PKRU is the hardware.

fpu_reset_fpstate() has two callers:

1) fpu__clear_user_states() for !FPU systems. For those PKRU is irrelevant

2) fpu_flush_thread() which is invoked from flush_thread(). flush_thread()
resets the hardware to the kernel restrictive default value.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/fpu/core.c | 22 ++++------------------
1 file changed, 4 insertions(+), 18 deletions(-)

--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -403,23 +403,6 @@ static inline unsigned int init_fpstate_
return sizeof(init_fpstate.xsave);
}

-/* Temporary workaround. Will be removed once PKRU and XSTATE are distangled. */
-static inline void pkru_set_default_in_xstate(struct xregs_state *xsave)
-{
- struct pkru_state *pk;
-
- if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
- return;
- /*
- * Force XFEATURE_PKRU to be set in the header otherwise
- * get_xsave_addr() does not work and it also needs to be set to
- * make XRSTOR(S) load it.
- */
- xsave->header.xfeatures |= XFEATURE_MASK_PKRU;
- pk = get_xsave_addr(xsave, XFEATURE_PKRU);
- pk->pkru = pkru_get_init_value();
-}
-
/*
* Reset current->fpu memory state to the init values.
*/
@@ -437,9 +420,12 @@ static void fpu_reset_fpstate(void)
*
* Do not use fpstate_init() here. Just copy init_fpstate which has
* the correct content already except for PKRU.
+ *
+ * PKRU handling does not rely on the xstate when restoring for
+ * user space as PKRU is eagerly written in switch_to() and
+ * flush_thread().
*/
memcpy(&fpu->state, &init_fpstate, init_fpstate_copy_size());
- pkru_set_default_in_xstate(&fpu->state.xsave);
set_thread_flag(TIF_NEED_FPU_LOAD);
fpregs_unlock();
}