x86/virt: Provide "nosnp" boot option for sev kernel command line

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.

  [ bp: Actually make it a HV-only cmdline option. ]

Co-developed-by: Eric Van Tassell <[email protected]>
Signed-off-by: Eric Van Tassell <[email protected]>
Signed-off-by: Pavan Kumar Paluri <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Tom Lendacky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: pvpk1994

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.

arch/x86/virt/svm/cmdline.c

@@ -10,6 +10,7 @@
 #include <linux/string.h>
 #include <linux/printk.h>
 #include <linux/cache.h>
+#include <linux/cpufeature.h>
 
 #include <asm/sev-common.h>
 
@@ -25,6 +26,17 @@ static int __init init_sev_config(char *str)
 			continue;
 		}
 
+		if (!strcmp(s, "nosnp")) {
+			if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) {
+				setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
+				cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
+				continue;
+			} else {
+				goto warn;
+			}
+		}
+
+warn:
 		pr_info("SEV command-line option '%s' was not recognized\n", s);
 	}