[PATCH 4/8] x86/spec-ctrl: Extend all SPEC_CTRL_{ENTER,EXIT}_* comments

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

... to better explain how they're used.

Doing so highlights that SPEC_CTRL_EXIT_TO_XEN is missing a VERW flush for the
corner case when e.g. an NMI hits late in an exit-to-guest path.

Leave a TODO, which will be addressed in subsequent patches which arrange for
DO_COND_VERW to be safe within SPEC_CTRL_EXIT_TO_XEN.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

This was decided not to be XSA-worthy, as guests can't usefully control when
IST events occur.
---
 xen/arch/x86/include/asm/spec_ctrl_asm.h | 35 ++++++++++++++++++++----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/include/asm/spec_ctrl_asm.h 
b/xen/arch/x86/include/asm/spec_ctrl_asm.h
index ee75f2bced42..77f6c35bb9c5 100644
--- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
+++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
@@ -218,7 +218,10 @@
     wrmsr
 .endm
 
-/* Use after an entry from PV context (syscall/sysenter/int80/int82/etc). */
+/*
+ * Used after a synchronous entry from PV context.  SYSCALL, SYSENTER, INT,
+ * etc.  Will always interrupt a guest speculation context.
+ */
 .macro SPEC_CTRL_ENTRY_FROM_PV
 /*
  * Requires %rsp=regs/cpuinfo, %rdx=0
@@ -233,7 +236,11 @@
         X86_FEATURE_SC_MSR_PV
 .endm
 
-/* Use in interrupt/exception context.  May interrupt Xen or PV context. */
+/*
+ * Used after a synchronous interrupt or exception.  May interrupt Xen or PV
+ * context, but will not interrupt Xen with a guest speculation context,
+ * outside of fatal error cases.
+ */
 .macro SPEC_CTRL_ENTRY_FROM_INTR
 /*
  * Requires %rsp=regs, %r14=stack_end, %rdx=0
@@ -248,7 +255,10 @@
         X86_FEATURE_SC_MSR_PV
 .endm
 
-/* Use when exiting to PV guest context. */
+/*
+ * Used when exiting from any entry context, back to PV context.  This
+ * includes from an IST entry which moved onto the primary stack.
+ */
 .macro SPEC_CTRL_EXIT_TO_PV
 /*
  * Requires %rax=spec_ctrl, %rsp=regs/info
@@ -260,7 +270,12 @@
 .endm
 
 /*
- * Use in IST interrupt/exception context.  May interrupt Xen or PV context.
+ * Used after an IST entry (i.e. needs special care, consider to be fully
+ * asynchronous with finding sane state).  May interrupt PV or Xen context,
+ * including other SPEC_CTRL_{ENTRY,EXIT}_* regions with unsanitised state.
+ *
+ * An IST entry which interrupts PV context moves onto the primary stack and
+ * leaves via SPEC_CTRL_EXIT_TO_PV, *not* SPEC_CTRL_EXIT_TO_XEN.
  */
 .macro SPEC_CTRL_ENTRY_FROM_INTR_IST
 /*
@@ -319,7 +334,14 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
     UNLIKELY_END(\@_serialise)
 .endm
 
-/* Use when exiting to Xen in IST context. */
+/*
+ * Use when exiting from any entry context, back to Xen context.  This
+ * includes returning to other SPEC_CTRL_{ENTRY,EXIT}_* regions with
+ * unsanitised state.
+ *
+ * Because we might have interrupted Xen beyond SPEC_CTRL_EXIT_TO_$GUEST, we
+ * must treat this as if it were an EXIT_TO_$GUEST case too.
+ */
 .macro SPEC_CTRL_EXIT_TO_XEN
 /*
  * Requires %rbx=stack_end
@@ -344,6 +366,9 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
     wrmsr
 
 .L\@_skip_sc_msr:
+
+    /* TODO VERW */
+
 .endm
 
 #endif /* __ASSEMBLY__ */
-- 
2.30.2