[PATCH v4 2/9] x86/sev: Save and print negotiated GHCB protocol version

[Vasant Karasulli]

From: Joerg Roedel <jroedel@xxxxxxx>

Save the results of the GHCB protocol negotiation into a data structure
and print information about versions supported and used to the kernel
log.

This is useful for debugging kexec issues in SEV-ES guests down the
road to quickly spot whether kexec is supported on the given host.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: Vasant Karasulli <vkarasulli@xxxxxxx>
---
 arch/x86/kernel/sev-shared.c | 33 +++++++++++++++++++++++++++++++--
 arch/x86/kernel/sev.c        |  8 ++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 1d24ec679915..c02a087c7945 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -17,6 +17,23 @@
 #define WARN(condition, format...) (!!(condition))
 #endif

+/*
+ * struct ghcb_info - Used to return GHCB protocol
+ *				   negotiation details.
+ *
+ * @hv_proto_min:	Minimum GHCB protocol version supported by Hypervisor
+ * @hv_proto_max:	Maximum GHCB protocol version supported by Hypervisor
+ * @vm_proto:		Protocol version the VM (this kernel) will use
+ */
+struct ghcb_info {
+	unsigned int hv_proto_min;
+	unsigned int hv_proto_max;
+	unsigned int vm_proto;
+};
+
+/* Negotiated GHCB protocol version */
+static struct ghcb_info ghcb_info __ro_after_init;
+
 /* I/O parameters for CPUID-related helpers */
 struct cpuid_leaf {
 	u32 fn;
@@ -152,12 +169,24 @@ static bool sev_es_negotiate_protocol(void)
 	if (GHCB_MSR_INFO(val) != GHCB_MSR_SEV_INFO_RESP)
 		return false;

-	if (GHCB_MSR_PROTO_MAX(val) < GHCB_PROTOCOL_MIN ||
-	    GHCB_MSR_PROTO_MIN(val) > GHCB_PROTOCOL_MAX)
+	/* Sanity check untrusted input */
+	if (GHCB_MSR_PROTO_MIN(val) > GHCB_MSR_PROTO_MAX(val))
 		return false;

+	/* Use maximum supported protocol version */
 	ghcb_version = min_t(size_t, GHCB_MSR_PROTO_MAX(val), GHCB_PROTOCOL_MAX);

+	/*
+	 * Hypervisor does not support any protocol version required for this
+	 * kernel.
+	 */
+	if (ghcb_version < GHCB_MSR_PROTO_MIN(val))
+		return false;
+
+	ghcb_info.hv_proto_min = GHCB_MSR_PROTO_MIN(val);
+	ghcb_info.hv_proto_max = GHCB_MSR_PROTO_MAX(val);
+	ghcb_info.vm_proto     = ghcb_version;
+
 	return true;
 }

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c67285824e82..179ab6eab0be 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1392,6 +1392,14 @@ void __init sev_es_init_vc_handling(void)

 	/* Secondary CPUs use the runtime #VC handler */
 	initial_vc_handler = (unsigned long)kernel_exc_vmm_communication;
+
+	/*
+	 * Print information about supported and negotiated GHCB protocol
+	 * versions.
+	 */
+	pr_info("Hypervisor GHCB protocol version support: min=%u max=%u\n",
+		ghcb_info.hv_proto_min, ghcb_info.hv_proto_max);
+	pr_info("Using GHCB protocol version %u\n", ghcb_info.vm_proto);
 }

 static void __init vc_early_forward_exception(struct es_em_ctxt *ctxt)
--
2.34.1