[PATCH Part1 RFC v4 11/36] x86/sev: Register GHCB memory when SEV-SNP is active

[Brijesh Singh <brijesh.singh@xxxxxxx>]

The SEV-SNP guest is required to perform GHCB GPA registration. This is
because the hypervisor may prefer that a guest use a consistent and/or
specific GPA for the GHCB associated with a vCPU. For more information,
see the GHCB specification section GHCB GPA Registration.

During the boot, init_ghcb() allocates a per-cpu GHCB page. On very first
VC exception, the exception handler switch to using the per-cpu GHCB page
allocated during the init_ghcb(). The GHCB page must be registered in
the current vcpu context.

Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---
 arch/x86/kernel/sev-internal.h | 12 ++++++++++++
 arch/x86/kernel/sev.c          | 29 +++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 arch/x86/kernel/sev-internal.h

diff --git a/arch/x86/kernel/sev-internal.h b/arch/x86/kernel/sev-internal.h
new file mode 100644
index 000000000000..0fb7324803b4
--- /dev/null
+++ b/arch/x86/kernel/sev-internal.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Forward declarations for sev-shared.c
+ *
+ * Author: Brijesh Singh <brijesh.singh@xxxxxxx>
+ */
+
+#ifndef __X86_SEV_INTERNAL_H__
+
+static void snp_register_ghcb_early(unsigned long paddr);
+
+#endif	/* __X86_SEV_INTERNAL_H__ */
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 504169f1966b..c7ff60b55bde 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -31,6 +31,8 @@
 #include <asm/smp.h>
 #include <asm/cpu.h>
 
+#include "sev-internal.h"
+
 #define DR7_RESET_VALUE        0x400
 
 /* For early boot hypervisor communication in SEV-ES enabled guests */
@@ -87,6 +89,13 @@ struct sev_es_runtime_data {
 	 * is currently unsupported in SEV-ES guests.
 	 */
 	unsigned long dr7;
+
+	/*
+	 * SEV-SNP requires that the GHCB must be registered before using it.
+	 * The flag below will indicate whether the GHCB is registered, if its
+	 * not registered then sev_es_get_ghcb() will perform the registration.
+	 */
+	bool snp_ghcb_registered;
 };
 
 struct ghcb_state {
@@ -194,6 +203,17 @@ void noinstr __sev_es_ist_exit(void)
 	this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], *(unsigned long *)ist);
 }
 
+static void snp_register_ghcb(struct sev_es_runtime_data *data, unsigned long paddr)
+{
+	if (data->snp_ghcb_registered)
+		return;
+
+	snp_register_ghcb_early(paddr);
+
+	data->snp_ghcb_registered = true;
+}
+
+
 /*
  * Nothing shall interrupt this code path while holding the per-CPU
  * GHCB. The backup GHCB is only for NMIs interrupting this path.
@@ -240,6 +260,10 @@ static noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
 		data->ghcb_active = true;
 	}
 
+	/* SEV-SNP guest requires that GHCB must be registered. */
+	if (sev_feature_enabled(SEV_SNP))
+		snp_register_ghcb(data, __pa(ghcb));
+
 	return ghcb;
 }
 
@@ -684,6 +708,10 @@ static bool __init setup_ghcb(void)
 	/* Alright - Make the boot-ghcb public */
 	boot_ghcb = &boot_ghcb_page;
 
+	/* SEV-SNP guest requires that GHCB GPA must be registered. */
+	if (sev_feature_enabled(SEV_SNP))
+		snp_register_ghcb_early(__pa(&boot_ghcb_page));
+
 	return true;
 }
 
@@ -773,6 +801,7 @@ static void __init init_ghcb(int cpu)
 
 	data->ghcb_active = false;
 	data->backup_ghcb_active = false;
+	data->snp_ghcb_registered = false;
 }
 
 void __init sev_es_init_vc_handling(void)
-- 
2.17.1