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

[tip-bot2 for Pavan Kumar Paluri]

The following commit has been merged into the x86/sev branch of tip:

Commit-ID:     2db67aaca578ec4998b78dc85e2af214bc2e2770
Gitweb:        https://git.kernel.org/tip/2db67aaca578ec4998b78dc85e2af214bc2e2770
Author:        Pavan Kumar Paluri <papaluri@xxxxxxx>
AuthorDate:    Mon, 14 Oct 2024 08:09:48 -05:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Tue, 15 Oct 2024 20:22:18 +02:00

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 <Eric.VanTassell@xxxxxxx>
Signed-off-by: Eric Van Tassell <Eric.VanTassell@xxxxxxx>
Signed-off-by: Pavan Kumar Paluri <papaluri@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Link: https://lore.kernel.org/r/20241014130948.1476946-3-papaluri@xxxxxxx
---
 Documentation/arch/x86/x86_64/boot-options.rst |  5 +++++
 arch/x86/virt/svm/cmdline.c                    | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/arch/x86/x86_64/boot-options.rst b/Documentation/arch/x86/x86_64/boot-options.rst
index 98d4805..d69e3cf 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 add4bae..affa275 100644
--- a/arch/x86/virt/svm/cmdline.c
+++ b/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);
 	}