[PATCH v6 11/17] KVM: arm64: Introduce hyp_dump_backtrace()

[Kalesh Singh]

In non-protected nVHE mode, unwinds and dumps the hypervisor backtrace
from EL1. This is possible beacause the host can directly access the
hypervisor stack pages in non-protected mode.

The nVHE backtrace is dumped on hyp_panic(), before panicking the host.

[  101.498183] kvm [377]: nVHE call trace:
[  101.498363] kvm [377]:  [<ffff8000090a6570>] __kvm_nvhe_hyp_panic+0xac/0xf8
[  101.499045] kvm [377]:  [<ffff8000090a65cc>] __kvm_nvhe_hyp_panic_bad_stack+0x10/0x10
[  101.499498] kvm [377]:  [<ffff8000090a61e4>] __kvm_nvhe_recursive_death+0x24/0x34
. . .
[  101.524929] kvm [377]:  [<ffff8000090a61e4>] __kvm_nvhe_recursive_death+0x24/0x34
[  101.525062] kvm [377]:  [<ffff8000090a61e4>] __kvm_nvhe_recursive_death+0x24/0x34
[  101.525195] kvm [377]:  [<ffff8000090a5de4>] __kvm_nvhe___kvm_vcpu_run+0x30/0x40c
[  101.525333] kvm [377]:  [<ffff8000090a8b64>] __kvm_nvhe_handle___kvm_vcpu_run+0x30/0x48
[  101.525468] kvm [377]:  [<ffff8000090a88b8>] __kvm_nvhe_handle_trap+0xc4/0x128
[  101.525602] kvm [377]:  [<ffff8000090a7864>] __kvm_nvhe___host_exit+0x64/0x64
[  101.525745] kvm [377]: ---[ end nVHE call trace ]---

Signed-off-by: Kalesh Singh <kaleshsingh at google.com>
---

Changes in v6:
  - Fix some typos in commit text and comments, per Fuad
  - Remove kvm_nvhe_print_backtrace_entry(), per Oliver
  - To make nVHE call trace delimiters consistent between protected and
    non-protected mode, factor it out into helpers, per Oliver
  - Change end delimiter to more match that of arm64 stacktrace
    (---[ end nVHE call trace ]---), per Oliver

Changes in v5:
  - Move code out from nvhe.h header to handle_exit.c, per Marc
  - Fix stacktrace symbolization when CONFIG_RAMDOMIZE_BASE is enabled,
    per Fuad
  - Use regular comments instead of doc comments, per Fuad

 arch/arm64/include/asm/stacktrace/nvhe.h | 17 ++++++
 arch/arm64/kvm/handle_exit.c             | 69 ++++++++++++++++++++++++
 2 files changed, 86 insertions(+)

diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
index 21082fd4a0b7..170fe7459f7c 100644
--- a/arch/arm64/include/asm/stacktrace/nvhe.h
+++ b/arch/arm64/include/asm/stacktrace/nvhe.h
@@ -16,6 +16,23 @@
 
 #include <asm/stacktrace/common.h>
 
+/*
+ * kvm_nvhe_unwind_init - Start an unwind from the given nVHE HYP fp and pc
+ *
+ * @state : unwind_state to initialize
+ * @fp    : frame pointer at which to start the unwinding.
+ * @pc    : program counter at which to start the unwinding.
+ */
+static inline void kvm_nvhe_unwind_init(struct unwind_state *state,
+					unsigned long fp,
+					unsigned long pc)
+{
+	unwind_init_common(state, NULL);
+
+	state->fp = fp;
+	state->pc = pc;
+}
+
 static inline bool on_hyp_stack(unsigned long sp, unsigned long size,
 				struct stack_info *info);
 
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index f66c0142b335..e83e6f735100 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -17,6 +17,7 @@
 #include <asm/kvm_emulate.h>
 #include <asm/kvm_mmu.h>
 #include <asm/debug-monitors.h>
+#include <asm/stacktrace/nvhe.h>
 #include <asm/traps.h>
 
 #include <kvm/arm_hypercalls.h>
@@ -318,6 +319,71 @@ void handle_exit_early(struct kvm_vcpu *vcpu, int exception_index)
 		kvm_handle_guest_serror(vcpu, kvm_vcpu_get_esr(vcpu));
 }
 
+/*
+ * kvm_nvhe_dump_backtrace_entry - Symbolize and print an nVHE backtrace entry
+ *
+ * @arg    : the hypervisor offset, used for address translation
+ * @where  : the program counter corresponding to the stack frame
+ */
+static bool kvm_nvhe_dump_backtrace_entry(void *arg, unsigned long where)
+{
+	unsigned long va_mask = GENMASK_ULL(vabits_actual - 1, 0);
+	unsigned long hyp_offset = (unsigned long)arg;
+
+	/* Mask tags and convert to kern addr */
+	where = (where & va_mask) + hyp_offset;
+	kvm_err(" [<%016lx>] %pB\n", where, (void *)(where + kaslr_offset()));
+
+	return true;
+}
+
+static inline void kvm_nvhe_dump_backtrace_start(void)
+{
+	kvm_err("nVHE call trace:\n");
+}
+
+static inline void kvm_nvhe_dump_backtrace_end(void)
+{
+	kvm_err("---[ end nVHE call trace ]---\n");
+}
+
+/*
+ * hyp_dump_backtrace - Dump the non-protected nVHE backtrace.
+ *
+ * @hyp_offset: hypervisor offset, used for address translation.
+ *
+ * The host can directly access HYP stack pages in non-protected
+ * mode, so the unwinding is done directly from EL1. This removes
+ * the need for shared buffers between host and hypervisor for
+ * the stacktrace.
+ */
+static void hyp_dump_backtrace(unsigned long hyp_offset)
+{
+	struct kvm_nvhe_stacktrace_info *stacktrace_info;
+	struct unwind_state state;
+
+	stacktrace_info = this_cpu_ptr_nvhe_sym(kvm_stacktrace_info);
+
+	kvm_nvhe_unwind_init(&state, stacktrace_info->fp, stacktrace_info->pc);
+
+	kvm_nvhe_dump_backtrace_start();
+	unwind(&state, kvm_nvhe_dump_backtrace_entry, (void *)hyp_offset);
+	kvm_nvhe_dump_backtrace_end();
+}
+
+/*
+ * kvm_nvhe_dump_backtrace - Dump KVM nVHE hypervisor backtrace.
+ *
+ * @hyp_offset: hypervisor offset, used for address translation.
+ */
+static void kvm_nvhe_dump_backtrace(unsigned long hyp_offset)
+{
+	if (is_protected_kvm_enabled())
+		return;
+	else
+		hyp_dump_backtrace(hyp_offset);
+}
+
 void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 					      u64 elr_virt, u64 elr_phys,
 					      u64 par, uintptr_t vcpu,
@@ -353,6 +419,9 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 				(void *)panic_addr);
 	}
 
+	/* Dump the nVHE hypervisor backtrace */
+	kvm_nvhe_dump_backtrace(hyp_offset);
+
 	/*
 	 * Hyp has panicked and we're going to handle that by panicking the
 	 * kernel. The kernel offset will be revealed in the panic so we're
-- 
2.37.1.359.gd136c6c3e2-goog