x86/mm: Add .bss..decrypted section to hold shared variables

kvmclock defines few static variables which are shared with the
hypervisor during the kvmclock initialization.

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 before sharing it.

Currently, we use kernel_physical_mapping_init() to split large pages
before clearing the C-bit on shared pages. But it fails when called from
the kvmclock initialization (mainly because the memblock allocator is
not ready that early during boot).

Add a __bss_decrypted section attribute which can be used when defining
such shared variable. The so-defined variables will be placed in the
.bss..decrypted section. This section will be mapped with C=0 early
during boot.

The .bss..decrypted section has a big chunk of memory that may be unused
when memory encryption is not active, free it when memory encryption is
not active.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Radim Krčmář<rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Link: https://lkml.kernel.org/r/1536932759-12905-2-git-send-email-brijesh.singh@amd.com

arch/x86/kernel/vmlinux.lds.S

@@ -65,6 +65,23 @@ jiffies_64 = jiffies;
 #define ALIGN_ENTRY_TEXT_BEGIN	. = ALIGN(PMD_SIZE);
 #define ALIGN_ENTRY_TEXT_END	. = ALIGN(PMD_SIZE);
 
+/*
+ * This section contains data which will be mapped as decrypted. Memory
+ * encryption operates on a page basis. Make this section PMD-aligned
+ * to avoid splitting the pages while mapping the section early.
+ *
+ * Note: We use a separate section so that only this section gets
+ * decrypted to avoid exposing more than we wish.
+ */
+#define BSS_DECRYPTED						\
+	. = ALIGN(PMD_SIZE);					\
+	__start_bss_decrypted = .;				\
+	*(.bss..decrypted);					\
+	. = ALIGN(PAGE_SIZE);					\
+	__start_bss_decrypted_unused = .;			\
+	. = ALIGN(PMD_SIZE);					\
+	__end_bss_decrypted = .;				\
+
 #else
 
 #define X86_ALIGN_RODATA_BEGIN
@@ -74,6 +91,7 @@ jiffies_64 = jiffies;
 
 #define ALIGN_ENTRY_TEXT_BEGIN
 #define ALIGN_ENTRY_TEXT_END
+#define BSS_DECRYPTED
 
 #endif
 
@@ -355,6 +373,7 @@ SECTIONS
 		__bss_start = .;
 		*(.bss..page_aligned)
 		*(.bss)
+		BSS_DECRYPTED
 		. = ALIGN(PAGE_SIZE);
 		__bss_stop = .;
 	}