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

DEFINE_PER_CPU_UNENCRYPTED can be used to define the per-cpu variables
which will be shared between guest and hypervisor. Currently, KVM defines
three variables (steal-time, apf_reason, and avic_eio) which are shared
with hypervisor.

Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: Borislav Petkov <b...@suse.de>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Tejun Heo <t...@kernel.org>
Cc: Christoph Lameter <c...@linux.com>
Cc: linux-a...@vger.kernel.org
Cc: x...@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Tom Lendacky <thomas.lenda...@amd.com>
Signed-off-by: Brijesh Singh <brijesh.si...@amd.com>
---
 include/asm-generic/vmlinux.lds.h | 11 +++++++++++
 include/linux/percpu-defs.h       | 15 +++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h 
b/include/asm-generic/vmlinux.lds.h
index 8acfc1e099e1..363858f43cbc 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -777,6 +777,16 @@
 #define INIT_RAM_FS
 #endif
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+#define PERCPU_UNENCRYPTED_SECTION                                     \
+       . = ALIGN(PAGE_SIZE);                                           \
+       *(.data..percpu..unencrypted)                                   \
+       . = ALIGN(PAGE_SIZE);
+#else
+#define PERCPU_UNENCRYPTED_SECTION
+#endif
+
+
 /*
  * Default discarded sections.
  *
@@ -815,6 +825,7 @@
        . = ALIGN(cacheline);                                           \
        *(.data..percpu)                                                \
        *(.data..percpu..shared_aligned)                                \
+       PERCPU_UNENCRYPTED_SECTION                                      \
        VMLINUX_SYMBOL(__per_cpu_end) = .;
 
 /**
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index 8f16299ca068..b2b99ad4b31d 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 unencrypted when memory encryption is enabled in the guest.
+ */
+#if defined(CONFIG_VIRTUALIZATION) && defined(CONFIG_AMD_MEM_ENCRYPT)
+
+#define DECLARE_PER_CPU_UNENCRYPTED(type, name)                                
\
+       DECLARE_PER_CPU_SECTION(type, name, "..unencrypted")
+
+#define DEFINE_PER_CPU_UNENCRYPTED(type, name)                         \
+       DEFINE_PER_CPU_SECTION(type, name, "..unencrypted")
+#else
+#define DEFINE_PER_CPU_UNENCRYPTED(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

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