[PATCH v6 2/2] x86 KVM:SVM: Provide "nosnp" boot option for sev kernel command line

[Pavan Kumar Paluri <papaluri@amd.com>]

Provide a "nosnp" kernel command line option to prevent enabling of the
RMP and SEV-SNP features in the host/hypervisor. Not initializing the
RMP removes system overhead associated with RMP checks.

Co-developed-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Pavan Kumar Paluri <papaluri@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 Documentation/arch/x86/x86_64/boot-options.rst | 5 +++++
 arch/x86/virt/svm/cmdline.c                    | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/Documentation/arch/x86/x86_64/boot-options.rst b/Documentation/arch/x86/x86_64/boot-options.rst
index 98d4805f0823..d69e3cfbdba5 100644
--- a/Documentation/arch/x86/x86_64/boot-options.rst
+++ b/Documentation/arch/x86/x86_64/boot-options.rst
@@ -305,3 +305,8 @@ The available options are:
 
    debug
      Enable debug messages.
+
+   nosnp
+     Do not enable SEV-SNP (applies to host/hypervisor only). Setting
+     'nosnp' avoids the RMP check overhead in memory accesses when
+     users do not want to run SEV-SNP guests.
diff --git a/arch/x86/virt/svm/cmdline.c b/arch/x86/virt/svm/cmdline.c
index 9640507342e0..313415d6f53f 100644
--- a/arch/x86/virt/svm/cmdline.c
+++ b/arch/x86/virt/svm/cmdline.c
@@ -11,6 +11,7 @@
 #include <linux/printk.h>
 
 #include <asm/sev-common.h>
+#include <asm/cpufeature.h>
 
 struct sev_config sev_cfg __read_mostly;
 
@@ -24,6 +25,12 @@ static int __init init_sev_config(char *str)
 			continue;
 		}
 
+		if (!strcmp(s, "nosnp")) {
+			setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
+			cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
+			continue;
+		}
+
 		pr_info("SEV command-line option '%s' was not recognized\n", s);
 	}
 
-- 
2.34.1