From: Joerg Roedel <jroedel@xxxxxxx>
Better explain why this code is necessary and what it is doing.
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
arch/x86/kernel/sev-es.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 0df38b185d53..79241bc45f25 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -127,14 +127,20 @@ static __always_inline bool on_vc_stack(unsigned long sp)
}
/*
- * This function handles the case when an NMI is raised in the #VC exception
- * handler entry code. In this case, the IST entry for #VC must be adjusted, so
- * that any subsequent #VC exception will not overwrite the stack contents of the
- * interrupted #VC handler.
+ * This function handles the case when an NMI is raised in the #VC
+ * exception handler entry code, before the #VC handler has switched off
+ * its IST stack. In this case, the IST entry for #VC must be adjusted,
+ * so that any nested #VC exception will not overwrite the stack
+ * contents of the interrupted #VC handler.
*
* The IST entry is adjusted unconditionally so that it can be also be
- * unconditionally adjusted back in sev_es_ist_exit(). Otherwise a nested
- * sev_es_ist_exit() call may adjust back the IST entry too early.
+ * unconditionally adjusted back in __sev_es_ist_exit(). Otherwise a
+ * nested sev_es_ist_exit() call may adjust back the IST entry too
+ * early.
+ *
+ * The __sev_es_ist_enter() and __sev_es_ist_exit() functions always run
+ * on the NMI IST stack, as they are only called from NMI handling code
+ * right now.
*/
void noinstr __sev_es_ist_enter(struct pt_regs *regs)
{
@@ -143,7 +149,10 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
/* Read old IST entry */
old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
- /* Make room on the IST stack */
+ /*
+ * Make room on the IST stack - Reserve 8 bytes to store the old
+ * IST entry.
+ */
if (on_vc_stack(regs->sp) &&
!user_mode(regs) &&
!from_syscall_gap(regs))
--
2.30.0
