[PATCH v3] xen/riscv: dump GPRs and CSRs on unexpected traps

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Provide additional context when an unexpected exception occurs by dumping
the relevant Supervisor, Virtual Supervisor (VS), and Hypervisor CSRs,
along with the general-purpose registers associated with the trap.

Dumping VS-mode CSRs in addition to host CSRs is beneficial when analysing
VS-mode traps. VSCAUSE, VSEPC, VSTVAL, and related VS state are required to
properly diagnose unexpected guest traps and potential hypervisor
misconfiguration.
For example, on an illegal-instruction exception the hardware may record
the faulting instruction in VSTVAL. If VSTVAL is zero, VSEPC should always
be inspected, and can be used together with objdump to identify the
faulting instruction. Dumping VSCAUSE is also useful when the guest does
not report it, or when the hypervisor redirects a trap to the guest using
VSCAUSE, VSTATUS, and VSTVEC, allowing verification that a subsequent trap
is not caused by incorrect VS state.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in v3:
 - Refactor the code dumping of CSRs and GPRs:
   - Introduce new macros
   - Re-group some registers when values are displayed.
 - Print all registers name in lower case.
 - Drop unnessary "Dumping CSRs", "Dumping GSRs" as it
   is clear based on names.
 - Update the commit message: add justification of dumping of
   some VS* registers.
 - Drop printing of VSSATP as I don't know usecase for now
   where it could be needed.
---
Changes in v2:
 - Address coments about print_csr() macros and dump_csrs().
 - Add dumping of GPRs pertaining to the trap.
---
 xen/arch/riscv/traps.c | 69 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/xen/arch/riscv/traps.c b/xen/arch/riscv/traps.c
index e9c967786312..7b9bcb97035f 100644
--- a/xen/arch/riscv/traps.c
+++ b/xen/arch/riscv/traps.c
@@ -99,12 +99,81 @@ static const char *decode_cause(unsigned long cause)
     return decode_trap_cause(cause);
 }
 
+static void dump_general_regs(const struct cpu_user_regs *regs)
+{
+#define GPR_LIST \
+    X(regs, ra, " ") X(regs, sp, "\n") \
+    X(regs, gp, " ") X(regs, tp, "\n") \
+    X(regs, t0, " ") X(regs, t1, "\n") \
+    X(regs, t2, " ") X(regs, s0, "\n") \
+    X(regs, s1, " ") X(regs, a0, "\n") \
+    X(regs, a1, " ") X(regs, a2, "\n") \
+    X(regs, a3, " ") X(regs, a4, "\n") \
+    X(regs, a5, " ") X(regs, a6, "\n") \
+    X(regs, a7, " ") X(regs, s2, "\n") \
+    X(regs, s3, " ") X(regs, s4, "\n") \
+    X(regs, s5, " ") X(regs, s6, "\n") \
+    X(regs, s7, " ") X(regs, s8, "\n") \
+    X(regs, s9, " ") X(regs, s10, "\n") \
+    X(regs, s11, " ") X(regs, t3, "\n") \
+    X(regs, t4, " ") X(regs, t4, "\n")
+
+#define X(regs, name, delim) \
+    printk("\t %-4s: %016lx" delim, #name, (regs)->name);
+
+    GPR_LIST
+
+#undef X
+#undef GPR_LIST
+}
+
+static void dump_csrs(unsigned long cause)
+{
+#define CSR_LIST \
+    X(stvec, CSR_STVEC, " ") X(vstvec, CSR_VSTVEC, "\n") \
+    X(sepc, CSR_SEPC, " ") X(vsepc, CSR_VSEPC, "\n") \
+    X(stval, CSR_STVAL, " ") X(vstval, CSR_VSTVAL, "\n") \
+    X(status, CSR_SSTATUS, " ") X(vsstatus, CSR_VSSTATUS, "\n") \
+    X(satp, CSR_SATP, "\n") \
+    X(scause, CSR_SCAUSE, " [%s]\n", \
+      decode_cause(v)) \
+    X(vscause, CSR_VSCAUSE, " [%s]\n\n", \
+      decode_cause(v)) \
+    X(hstatus, CSR_HSTATUS, " [%s%s%s%s%s%s ]\n", \
+      (v & HSTATUS_VTSR) ? " VTSR" : "", \
+      (v & HSTATUS_VTVM) ? " VTVM" : "", \
+      (v & HSTATUS_HU)   ? " HU"   : "", \
+      (v & HSTATUS_SPVP) ? " SPVP" : "", \
+      (v & HSTATUS_SPV)  ? " SPV"  : "", \
+      (v & HSTATUS_GVA)  ? " GVA"  : "") \
+    X(hgatp, CSR_HGATP, "\n") \
+    X(htval, CSR_HTVAL, "\n") \
+    X(htinst, CSR_HTINST, "\n") \
+    X(hedeleg, CSR_HEDELEG, "\n") \
+    X(hideleg, CSR_HIDELEG, "\n") \
+    X(hstateen0, CSR_HSTATEEN0, "\n\n")
+
+#define X(name, csr, fmt, ...) \
+do { \
+    unsigned long v = csr_read(csr); \
+    printk("\t %-10s: %016lx" fmt, #name, v, ##__VA_ARGS__); \
+} while (0);
+
+    CSR_LIST
+
+#undef X
+#undef CSR_LIST
+}
+
 static void do_unexpected_trap(const struct cpu_user_regs *regs)
 {
     unsigned long cause = csr_read(CSR_SCAUSE);
 
     printk("Unhandled exception: %s\n", decode_cause(cause));
 
+    dump_csrs(cause);
+    dump_general_regs(regs);
+
     die();
 }
 
-- 
2.52.0