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@@ -294,13 +294,29 @@ sysenter_past_esp:
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pushl $__USER_DS /* pt_regs->ss */
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pushl %ebp /* pt_regs->sp (stashed in bp) */
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pushfl /* pt_regs->flags (except IF = 0) */
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- ASM_CLAC /* Clear AC after saving FLAGS */
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orl $X86_EFLAGS_IF, (%esp) /* Fix IF */
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pushl $__USER_CS /* pt_regs->cs */
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pushl $0 /* pt_regs->ip = 0 (placeholder) */
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pushl %eax /* pt_regs->orig_ax */
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SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest */
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+ /*
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+ * SYSENTER doesn't filter flags, so we need to clear NT and AC
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+ * ourselves. To save a few cycles, we can check whether
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+ * either was set instead of doing an unconditional popfq.
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+ * This needs to happen before enabling interrupts so that
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+ * we don't get preempted with NT set.
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+ *
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+ * NB.: .Lsysenter_fix_flags is a label with the code under it moved
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+ * out-of-line as an optimization: NT is unlikely to be set in the
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+ * majority of the cases and instead of polluting the I$ unnecessarily,
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+ * we're keeping that code behind a branch which will predict as
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+ * not-taken and therefore its instructions won't be fetched.
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+ */
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+ testl $X86_EFLAGS_NT|X86_EFLAGS_AC, PT_EFLAGS(%esp)
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+ jnz .Lsysenter_fix_flags
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+.Lsysenter_flags_fixed:
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+
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/*
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* User mode is traced as though IRQs are on, and SYSENTER
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* turned them off.
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@@ -339,6 +355,11 @@ sysenter_past_esp:
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.popsection
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_ASM_EXTABLE(1b, 2b)
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PTGS_TO_GS_EX
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+
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+.Lsysenter_fix_flags:
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+ pushl $X86_EFLAGS_FIXED
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+ popfl
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+ jmp .Lsysenter_flags_fixed
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ENDPROC(entry_SYSENTER_32)
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# system call handler stub
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