[Part1 PATCH v5 15/17] percpu: Introduce DEFINE_PER_CPU_DECRYPTED

[Brijesh Singh]

KVM guest defines three per-CPU variables (steal-time, apf_reason, and
avic_eio) 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.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
---
 include/asm-generic/vmlinux.lds.h | 19 +++++++++++++++++++
 include/linux/percpu-defs.h       | 15 +++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 8acfc1e099e1..2dd54c3e1a8a 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -778,6 +778,24 @@
 #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.
  *
  * Some archs want to discard exit text/data at runtime rather than
@@ -815,6 +833,7 @@
 	. = ALIGN(cacheline);						\
 	*(.data..percpu)						\
 	*(.data..percpu..shared_aligned)				\
+	PERCPU_DECRYPTED_SECTION					\
 	VMLINUX_SYMBOL(__per_cpu_end) = .;
 
 /**
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index 8f16299ca068..2d2096ba1cfe 100644
--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -173,6 +173,21 @@
 	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
  * noop if __CHECKER__.
-- 
2.9.5