[PATCH Part1 RFC v2 13/20] x86/sev: Register GHCB memory when SEV-SNP is active

[Brijesh Singh]

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.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 8c8c939a1754..e6819f170ec4 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -88,6 +88,13 @@ 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 {
@@ -100,6 +107,9 @@ DEFINE_STATIC_KEY_FALSE(sev_es_enable_key);
 /* Needed in vc_early_forward_exception */
 void do_early_exception(struct pt_regs *regs, int trapnr);
 
+/* Defined in sev-shared.c */
+static void snp_register_ghcb(unsigned long paddr);
+
 static void __init setup_vc_stacks(int cpu)
 {
 	struct sev_es_runtime_data *data;
@@ -218,6 +228,12 @@ static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
 		data->ghcb_active = true;
 	}
 
+	/* SEV-SNP guest requires that GHCB must be registered before using it. */
+	if (sev_snp_active() && !data->snp_ghcb_registered) {
+		snp_register_ghcb(__pa(ghcb));
+		data->snp_ghcb_registered = true;
+	}
+
 	return ghcb;
 }
 
@@ -622,6 +638,10 @@ static bool __init sev_es_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_snp_active())
+		snp_register_ghcb(__pa(&boot_ghcb_page));
+
 	return true;
 }
 
@@ -711,6 +731,7 @@ 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