[PATCH v2 1/1] BaseLib:Fix RISC-V Supervisor mode (S-Mode) trap handler reentry issue.

["Abner Chang" <abner.chang@hpe.com>]

While RISC-V hart is trapped into S-Mode, the S-Mode interrupt
CSR (SIE) is disabled by RISC-V hart. However the (SIE) is enabled
again by RestoreTPL, this causes the second S-Mode trap is triggered
by the machine mode (M-Mode)timer interrupt redirection. The SRET
instruction clear Supervisor Previous Privilege (SPP) to zero
(User mode) in the second S-Mode interrupt according to the RISC-V
spec. Above brings hart to the user mode (U-Mode) when execute
SRET in the nested S-Mode interrupt handler because SPP is set to
User Mode in the second interrupt. Afterward, system runs in U-Mode
and any accesses to S-Mode CSR causes the invalid instruction exception.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
---
 .../Library/BaseLib/RiscV64/RiscVInterrupt.S  | 45 ++++++++++++++++---
 1 file changed, 38 insertions(+), 7 deletions(-)

diff --git a/MdePkg/Library/BaseLib/RiscV64/RiscVInterrupt.S b/MdePkg/Library/BaseLib/RiscV64/RiscVInterrupt.S
index 766fcfb9cb..e821124781 100644
--- a/MdePkg/Library/BaseLib/RiscV64/RiscVInterrupt.S
+++ b/MdePkg/Library/BaseLib/RiscV64/RiscVInterrupt.S
@@ -12,21 +12,52 @@ ASM_GLOBAL ASM_PFX(RiscVDisableSupervisorModeInterrupts)
 ASM_GLOBAL ASM_PFX(RiscVEnableSupervisorModeInterrupt)
 ASM_GLOBAL ASM_PFX(RiscVGetSupervisorModeInterrupts)
 
-# define  MSTATUS_SIE    0x00000002
-# define  CSR_SSTATUS    0x100
+#define  SSTATUS_SIE                 0x00000002
+#define  CSR_SSTATUS                 0x100
+#define  SSTATUS_SPP_BIT_POSITION    8
 
+//
+// This routine disables supervisor mode interrupt
+//
 ASM_PFX(RiscVDisableSupervisorModeInterrupts):
-  li   a1, MSTATUS_SIE
-  csrc CSR_SSTATUS, a1
+  add   sp, sp, -(__SIZEOF_POINTER__)
+  sd    a1, (sp)
+  li    a1, SSTATUS_SIE
+  csrc  CSR_SSTATUS, a1
+  ld    a1, (sp)
+  add   sp, sp, (__SIZEOF_POINTER__)
   ret
 
+//
+// This routine enables supervisor mode interrupt
+//
 ASM_PFX(RiscVEnableSupervisorModeInterrupt):
-  li   a1, MSTATUS_SIE
-  csrs CSR_SSTATUS, a1
+  add   sp, sp, -2*(__SIZEOF_POINTER__)
+  sd    a0, (0*__SIZEOF_POINTER__)(sp)
+  sd    a1, (1*__SIZEOF_POINTER__)(sp)
+
+  csrr  a0, CSR_SSTATUS
+  and   a0, a0, (1 << SSTATUS_SPP_BIT_POSITION)
+  bnez  a0, InTrap      // We are in supervisor mode (SMode)
+                        // trap handler.
+                        // Skip enabling SIE becasue SIE
+                        // is set to disabled by RISC-V hart
+                        // when the trap takes hart to SMode.
+
+  li    a1, SSTATUS_SIE
+  csrs  CSR_SSTATUS, a1
+InTrap:
+  ld    a0, (0*__SIZEOF_POINTER__)(sp)
+  ld    a1, (1*__SIZEOF_POINTER__)(sp)
+  add   sp, sp, 2*(__SIZEOF_POINTER__)
   ret
 
+//
+// This routine returns supervisor mode interrupt
+// status.
+//
 ASM_PFX(RiscVGetSupervisorModeInterrupts):
   csrr a0, CSR_SSTATUS
-  andi a0, a0, MSTATUS_SIE
+  andi a0, a0, SSTATUS_SIE
   ret
 
-- 
2.25.0