percpu: Introduce DEFINE_PER_CPU_DECRYPTED

KVM guest defines three per-CPU variables (steal-time, apf_reason, and
kvm_pic_eoi) which are shared between a guest and a hypervisor.

When SEV is active, memory is encrypted with a guest-specific key, and if
the guest OS wants to share the memory region with the hypervisor then it
must clear the C-bit (i.e set decrypted) before sharing it.

DEFINE_PER_CPU_DECRYPTED can be used to define the per-CPU variables
which will be shared between a guest and a hypervisor.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: linux-arch@vger.kernel.org
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: kvm@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christoph Lameter <cl@linux.com>
Link: https://lkml.kernel.org/r/20171020143059.3291-16-brijesh.singh@amd.com

include/asm-generic/vmlinux.lds.h

@@ -777,6 +777,24 @@
 #define INIT_RAM_FS
 #endif
 
+/*
+ * Memory encryption operates on a page basis. Since we need to clear
+ * the memory encryption mask for this section, it needs to be aligned
+ * on a page boundary and be a page-size multiple in length.
+ *
+ * Note: We use a separate section so that only this section gets
+ * decrypted to avoid exposing more than we wish.
+ */
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+#define PERCPU_DECRYPTED_SECTION					\
+	. = ALIGN(PAGE_SIZE);						\
+	*(.data..percpu..decrypted)					\
+	. = ALIGN(PAGE_SIZE);
+#else
+#define PERCPU_DECRYPTED_SECTION
+#endif
+
+
 /*
  * Default discarded sections.
  *
@@ -815,6 +833,7 @@
 	. = ALIGN(cacheline);						\
 	*(.data..percpu)						\
 	*(.data..percpu..shared_aligned)				\
+	PERCPU_DECRYPTED_SECTION					\
 	VMLINUX_SYMBOL(__per_cpu_end) = .;
 
 /**

include/linux/percpu-defs.h

@@ -172,6 +172,21 @@
 #define DEFINE_PER_CPU_READ_MOSTLY(type, name)				\
 	DEFINE_PER_CPU_SECTION(type, name, "..read_mostly")
 
+/*
+ * Declaration/definition used for per-CPU variables that should be accessed
+ * as decrypted when memory encryption is enabled in the guest.
+ */
+#if defined(CONFIG_VIRTUALIZATION) && defined(CONFIG_AMD_MEM_ENCRYPT)
+
+#define DECLARE_PER_CPU_DECRYPTED(type, name)				\
+	DECLARE_PER_CPU_SECTION(type, name, "..decrypted")
+
+#define DEFINE_PER_CPU_DECRYPTED(type, name)				\
+	DEFINE_PER_CPU_SECTION(type, name, "..decrypted")
+#else
+#define DEFINE_PER_CPU_DECRYPTED(type, name)	DEFINE_PER_CPU(type, name)
+#endif
+
 /*
  * Intermodule exports for per-CPU variables.  sparse forgets about
  * address space across EXPORT_SYMBOL(), change EXPORT_SYMBOL() to