[xen stable-4.16] x86/spec-ctrl: Improve all SPEC_CTRL_{ENTER,EXIT}_* comments

[patchbot@xxxxxxx]

commit 6cc49c355e952f4ff564c6b817e7eff57c5a02c7
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Aug 30 20:11:50 2023 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Sep 19 19:38:04 2023 +0100

    x86/spec-ctrl: Improve all SPEC_CTRL_{ENTER,EXIT}_* comments
    
    ... 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
    VERW flushing to be safe within SPEC_CTRL_EXIT_TO_XEN.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 45f00557350dc7d0756551069803fc49c29184ca)
---
 xen/include/asm-x86/spec_ctrl_asm.h | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/xen/include/asm-x86/spec_ctrl_asm.h 
b/xen/include/asm-x86/spec_ctrl_asm.h
index 94ed5dc880..9c397f7cbd 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -235,7 +235,10 @@
     wrmsr
 .endm
 
-/* Use after an entry from PV context (syscall/sysenter/int80/int82/etc). */
+/*
+ * Used after an entry from PV context: SYSCALL, SYSENTER, INT,
+ * etc.  There is always a guest speculation state in context.
+ */
 .macro SPEC_CTRL_ENTRY_FROM_PV
 /*
  * Requires %rsp=regs/cpuinfo, %rdx=0
@@ -250,7 +253,11 @@
         X86_FEATURE_SC_MSR_PV
 .endm
 
-/* Use in interrupt/exception context.  May interrupt Xen or PV context. */
+/*
+ * Used after an exception or maskable interrupt, hitting Xen or PV context.
+ * There will either be a guest speculation context, or (barring fatal
+ * exceptions) a well-formed Xen speculation context.
+ */
 .macro SPEC_CTRL_ENTRY_FROM_INTR
 /*
  * Requires %rsp=regs, %r14=stack_end, %rdx=0
@@ -265,7 +272,10 @@
         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
@@ -277,7 +287,13 @@
 .endm
 
 /*
- * Use in IST interrupt/exception context.  May interrupt Xen or PV context.
+ * Used after an IST entry hitting Xen or PV context.  Special care is needed,
+ * because when hitting Xen context, there may not be a well-formed
+ * speculation context.  (i.e. it can hit in the middle of
+ * SPEC_CTRL_{ENTRY,EXIT}_* regions.)
+ *
+ * An IST entry which hits 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
 /*
@@ -336,7 +352,14 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
     UNLIKELY_END(\@_serialise)
 .endm
 
-/* Use when exiting to Xen context. */
+/*
+ * Use when exiting from any entry context, back to Xen context.  This
+ * includes returning to other SPEC_CTRL_{ENTRY,EXIT}_* regions with an
+ * incomplete speculation context.
+ *
+ * Because we might have interrupted Xen beyond SPEC_CTRL_EXIT_TO_$GUEST, we
+ * need to treat this as if it were an EXIT_TO_$GUEST case too.
+ */
 .macro SPEC_CTRL_EXIT_TO_XEN
 /*
  * Requires %rbx=stack_end
@@ -361,6 +384,9 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
     wrmsr
 
 .L\@_skip_sc_msr:
+
+    /* TODO VERW */
+
 .endm
 
 #endif /* __ASSEMBLY__ */
--
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