[PATCH v7 0/2] nosnp sev command line support

[PATCH v7 0/2] nosnp sev command line support

[Pavan Kumar Paluri]

Provide "nosnp" boot option via "sev=nosnp" kernel command line to
prevent SEV-SNP [1] capable host kernel from enabling SEV-SNP and
initializing Reverse Map Table (RMP)

Setting 'nosnp' avoids the RMP check overhead in memory accesses when
users do not want to run SEV-SNP guests.

On providing sev=nosnp via kernel command line:
cat /sys/module/kvm_amd/parameters/sev_snp should be "N".

The patchset is based on tip/master.

Reference:
[1] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf

Changelog:
=========
v6:
  * <linux/cache.h> was not included in virt/svm/cmdline.c, which was 
    breaking the progressive build. Fix this issue (Boris)
  * Link: https://lore.kernel.org/all/20241010121455.15795-1-papaluri@amd.com/
 
v5:
  * Update cover-letter and Documentation to include information on why
    nosnp command line option is required (Dave Hansen)
  * Remove <asm/cache.h> stray header introduced in the previous
    versions because of __read_mostly attribute that is now moved into
    virt/svm/cmdline.c
  * Link: https://lore.kernel.org/all/20240930231102.123403-1-papaluri@amd.com/

v4:
  * Move __read_mostly attribute to place where sev_cfg is declared (Tom)
  * Link: https://lore.kernel.org/all/20240922033626.29038-1-papaluri@amd.com/


Pavan Kumar Paluri (2):
  x86, KVM:SVM: Move sev specific parsing into arch/x86/virt/svm
  x86 KVM:SVM: Provide "nosnp" boot option for sev kernel command line

 .../arch/x86/x86_64/boot-options.rst          |  5 +++
 arch/x86/coco/sev/core.c                      | 44 -------------------
 arch/x86/include/asm/sev-common.h             | 27 ++++++++++++
 arch/x86/virt/svm/Makefile                    |  1 +
 arch/x86/virt/svm/cmdline.c                   | 40 +++++++++++++++++
 5 files changed, 73 insertions(+), 44 deletions(-)
 create mode 100644 arch/x86/virt/svm/cmdline.c


base-commit: 00d91979d23c88d3f50870e22fc9cec3f5e26a2a
-- 
2.34.1

[PATCH v7 1/2] x86, KVM:SVM: Move sev specific parsing into arch/x86/virt/svm

[Pavan Kumar Paluri]

Move SEV specific kernel command line option parsing support from
arch/x86/coco/sev/core.c to arch/x86/virt/svm/cmdline.c so that both
host and guest related SEV command line options can be supported.

No functional changes intended.

Signed-off-by: Pavan Kumar Paluri <papaluri@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/coco/sev/core.c          | 44 -------------------------------
 arch/x86/include/asm/sev-common.h | 27 +++++++++++++++++++
 arch/x86/virt/svm/Makefile        |  1 +
 arch/x86/virt/svm/cmdline.c       | 33 +++++++++++++++++++++++
 4 files changed, 61 insertions(+), 44 deletions(-)
 create mode 100644 arch/x86/virt/svm/cmdline.c

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index de1df0cb45da..ff19e805e7a1 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -141,33 +141,6 @@ static DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
 static DEFINE_PER_CPU(struct svsm_ca *, svsm_caa);
 static DEFINE_PER_CPU(u64, svsm_caa_pa);
 
-struct sev_config {
-	__u64 debug		: 1,
-
-	      /*
-	       * Indicates when the per-CPU GHCB has been created and registered
-	       * and thus can be used by the BSP instead of the early boot GHCB.
-	       *
-	       * For APs, the per-CPU GHCB is created before they are started
-	       * and registered upon startup, so this flag can be used globally
-	       * for the BSP and APs.
-	       */
-	      ghcbs_initialized	: 1,
-
-	      /*
-	       * Indicates when the per-CPU SVSM CA is to be used instead of the
-	       * boot SVSM CA.
-	       *
-	       * For APs, the per-CPU SVSM CA is created as part of the AP
-	       * bringup, so this flag can be used globally for the BSP and APs.
-	       */
-	      use_cas		: 1,
-
-	      __reserved	: 61;
-};
-
-static struct sev_config sev_cfg __read_mostly;
-
 static __always_inline bool on_vc_stack(struct pt_regs *regs)
 {
 	unsigned long sp = regs->sp;
@@ -2374,23 +2347,6 @@ static int __init report_snp_info(void)
 }
 arch_initcall(report_snp_info);
 
-static int __init init_sev_config(char *str)
-{
-	char *s;
-
-	while ((s = strsep(&str, ","))) {
-		if (!strcmp(s, "debug")) {
-			sev_cfg.debug = true;
-			continue;
-		}
-
-		pr_info("SEV command-line option '%s' was not recognized\n", s);
-	}
-
-	return 1;
-}
-__setup("sev=", init_sev_config);
-
 static void update_attest_input(struct svsm_call *call, struct svsm_attest_call *input)
 {
 	/* If (new) lengths have been returned, propagate them up */
diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index 98726c2b04f8..50f5666938c0 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -220,4 +220,31 @@ struct snp_psc_desc {
 #define GHCB_ERR_INVALID_INPUT		5
 #define GHCB_ERR_INVALID_EVENT		6
 
+struct sev_config {
+	__u64 debug		: 1,
+
+	      /*
+	       * Indicates when the per-CPU GHCB has been created and registered
+	       * and thus can be used by the BSP instead of the early boot GHCB.
+	       *
+	       * For APs, the per-CPU GHCB is created before they are started
+	       * and registered upon startup, so this flag can be used globally
+	       * for the BSP and APs.
+	       */
+	      ghcbs_initialized	: 1,
+
+	      /*
+	       * Indicates when the per-CPU SVSM CA is to be used instead of the
+	       * boot SVSM CA.
+	       *
+	       * For APs, the per-CPU SVSM CA is created as part of the AP
+	       * bringup, so this flag can be used globally for the BSP and APs.
+	       */
+	      use_cas		: 1,
+
+	      __reserved	: 61;
+};
+
+extern struct sev_config sev_cfg;
+
 #endif
diff --git a/arch/x86/virt/svm/Makefile b/arch/x86/virt/svm/Makefile
index ef2a31bdcc70..eca6d71355fa 100644
--- a/arch/x86/virt/svm/Makefile
+++ b/arch/x86/virt/svm/Makefile
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
 
 obj-$(CONFIG_KVM_AMD_SEV) += sev.o
+obj-$(CONFIG_CPU_SUP_AMD) += cmdline.o
diff --git a/arch/x86/virt/svm/cmdline.c b/arch/x86/virt/svm/cmdline.c
new file mode 100644
index 000000000000..add4bae3ebef
--- /dev/null
+++ b/arch/x86/virt/svm/cmdline.c
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * AMD SVM-SEV command line parsing support
+ *
+ * Copyright (C) 2023 - 2024 Advanced Micro Devices, Inc.
+ *
+ * Author: Michael Roth <michael.roth@amd.com>
+ */
+
+#include <linux/string.h>
+#include <linux/printk.h>
+#include <linux/cache.h>
+
+#include <asm/sev-common.h>
+
+struct sev_config sev_cfg __read_mostly;
+
+static int __init init_sev_config(char *str)
+{
+	char *s;
+
+	while ((s = strsep(&str, ","))) {
+		if (!strcmp(s, "debug")) {
+			sev_cfg.debug = true;
+			continue;
+		}
+
+		pr_info("SEV command-line option '%s' was not recognized\n", s);
+	}
+
+	return 1;
+}
+__setup("sev=", init_sev_config);
-- 
2.34.1

[PATCH v7 2/2] x86 KVM:SVM: Provide "nosnp" boot option for sev kernel command line

[Pavan Kumar Paluri]

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 add4bae3ebef..13e04547f4df 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 +26,12 @@ 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

Re: [PATCH v7 2/2] x86 KVM:SVM: Provide "nosnp" boot option for sev kernel command line

[Borislav Petkov]

On Mon, Oct 14, 2024 at 08:09:48AM -0500, Pavan Kumar Paluri wrote:
> @@ -25,6 +26,12 @@ 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;
> +		}

Well, if it is a HV-only option, then it better be such:

@@ -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);
 	}
 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH v7 2/2] x86 KVM:SVM: Provide "nosnp" boot option for sev kernel command line

[Paluri, PavanKumar]

Hello Boris,

On 10/14/2024 10:40 AM, Borislav Petkov wrote:
> On Mon, Oct 14, 2024 at 08:09:48AM -0500, Pavan Kumar Paluri wrote:
>> @@ -25,6 +26,12 @@ 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;
>> +		}
> 
> Well, if it is a HV-only option, then it better be such:
> 
> @@ -25,6 +26,17 @@ static int __init init_sev_config(char *str)
>  			continue;
>  		}
>  
> +		if (!strcmp(s, "nosnp")) {
> +			if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) {

Sure, it does make sense to add this check since nosnp is only a HV option.

Thanks,
Pavan

> +				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);
>  	}
>  
>