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Subject[RFC 08/17] x86/asm/64: De-Xen-ify our NMI code
Date
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.

XXX: Juergen: could you write and test the tiny patch needed to
make Xen PV have a xen_nmi entry that handles NMIs? I don't know
how to test it.

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 | 41 +++++++++++++++++------------------------
1 file changed, 17 insertions(+), 24 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a9e318f7cc9b..c81e05fb999e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1171,21 +1171,12 @@ 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)
/*
- * Fix up the exception frame if we're on Xen.
- * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most
- * one value to the stack on native, so it may clobber the rdx
- * scratch slot, but it won't clobber any of the important
- * slots past it.
- *
- * Xen is a different story, because the Xen frame itself overlaps
- * the "NMI executing" variable.
- */
- PARAVIRT_ADJUST_EXCEPTION_FRAME
-
- /*
* We allow breakpoints in NMIs. If a breakpoint occurs, then
* the iretq it performs will take us out of NMI context.
* This means that we can have nested NMIs where the next
@@ -1240,7 +1231,7 @@ 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
@@ -1402,7 +1393,7 @@ 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. */
@@ -1432,7 +1423,7 @@ first_nmi:
pushfq /* RFLAGS */
pushq $__KERNEL_CS /* CS */
pushq $1f /* RIP */
- INTERRUPT_RETURN /* continues at repeat_nmi below */
+ iretq /* continues at repeat_nmi below */
1:
#endif

@@ -1502,20 +1493,22 @@ 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)
--
2.13.5
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