KVM: arm64: Allocate shared pKVM hyp stacktrace buffers

In protected nVHE mode the host cannot directly access
hypervisor memory, so we will dump the hypervisor stacktrace
to a shared buffer with the host.

The minimum size for the buffer required, assuming the min frame
size of [x29, x30] (2 * sizeof(long)), is half the combined size of
the hypervisor and overflow stacks plus an additional entry to
delimit the end of the stacktrace.

The stacktrace buffers are used later in the series to dump the
nVHE hypervisor stacktrace when using protected-mode.
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220726073750.3219117-14-kaleshsingh@google.com

arch/arm64/include/asm/memory.h

@@ -113,6 +113,14 @@
 
 #define OVERFLOW_STACK_SIZE	SZ_4K
 
+/*
+ * With the minimum frame size of [x29, x30], exactly half the combined
+ * sizes of the hyp and overflow stacks is the maximum size needed to
+ * save the unwinded stacktrace; plus an additional entry to delimit the
+ * end.
+ */
+#define NVHE_STACKTRACE_SIZE	((OVERFLOW_STACK_SIZE + PAGE_SIZE) / 2 + sizeof(long))
+
 /*
  * Alignment of kernel segments (e.g. .text, .data).
  *

arch/arm64/kvm/hyp/nvhe/stacktrace.c

@@ -34,6 +34,10 @@ static void hyp_prepare_backtrace(unsigned long fp, unsigned long pc)
 	stacktrace_info->pc = pc;
 }
 
+#ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
+DEFINE_PER_CPU(unsigned long [NVHE_STACKTRACE_SIZE/sizeof(long)], pkvm_stacktrace);
+#endif /* CONFIG_PROTECTED_NVHE_STACKTRACE */
+
 /*
  * kvm_nvhe_prepare_backtrace - prepare to dump the nVHE backtrace
  *