[PATCH 09/18] x86/asm/64: De-Xen-ify our NMI code

Andy Lutomirski
2017-10-26T08:27:42Z

Xen PV is fundamentally incompatible with our fancy NMI code: it
doesn't use IST at all, and Xen entries clobber two stack slots
below the hardware frame.

Drop Xen PV support from our NMI code entirely.

Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/entry_64.S | 30 ++++++++++++++++++------------
1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index bf650fad94ce..b46c726fa750 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1236,9 +1236,13 @@ ENTRY(error_exit)
jmp retint_user
END(error_exit)

-/* Runs on exception stack */
+/*
+ * Runs on exception stack. Xen PV does not go through this path at all,
+ * so we can use real assembly here.
+ */
ENTRY(nmi)
UNWIND_HINT_IRET_REGS
+
/*
* We allow breakpoints in NMIs. If a breakpoint occurs, then
* the iretq it performs will take us out of NMI context.
@@ -1296,7 +1300,7 @@ ENTRY(nmi)
* stacks lest we corrupt the "NMI executing" variable.
*/

- SWAPGS_UNSAFE_STACK
+ swapgs
cld
movq %rsp, %rdx
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
@@ -1461,7 +1465,7 @@ nested_nmi_out:
popq %rdx

/* We are returning to kernel mode, so this cannot result in a fault. */
- INTERRUPT_RETURN
+ iretq

first_nmi:
/* Restore rdx. */
@@ -1492,7 +1496,7 @@ first_nmi:
pushfq /* RFLAGS */
pushq $__KERNEL_CS /* CS */
pushq $1f /* RIP */
- INTERRUPT_RETURN /* continues at repeat_nmi below */
+ iretq /* continues at repeat_nmi below */
UNWIND_HINT_IRET_REGS
1:
#endif
@@ -1564,20 +1568,22 @@ nmi_restore:
/*
* Clear "NMI executing". Set DF first so that we can easily
* distinguish the remaining code between here and IRET from
- * the SYSCALL entry and exit paths. On a native kernel, we
- * could just inspect RIP, but, on paravirt kernels,
- * INTERRUPT_RETURN can translate into a jump into a
- * hypercall page.
+ * the SYSCALL entry and exit paths.
+ *
+ * We arguably should just inspect RIP instead, but I (Andy) wrote
+ * this code when I had the misapprehension that Xen PV supported
+ * NMIs, and Xen PV would break that approach.
*/
std
movq $0, 5*8(%rsp) /* clear "NMI executing" */

/*
- * INTERRUPT_RETURN reads the "iret" frame and exits the NMI
- * stack in a single instruction. We are returning to kernel
- * mode, so this cannot result in a fault.
+ * iretq reads the "iret" frame and exits the NMI stack in a
+ * single instruction. We are returning to kernel mode, so this
+ * cannot result in a fault. Similarly, we don't need to worry
+ * about espfix64 on the way back to kernel mode.
*/
- INTERRUPT_RETURN
+ iretq
END(nmi)

ENTRY(ignore_sysret)
--
2.13.6