There are currently 4 mitigations that use VERW to flush different cpu
buffers. This can cause confusion when trying to disable all the
different VERW mitigations. Simplify enabling/disabling these
mitigations by creating a single parameter for controlling them.

Future work will focus on combining similar code used in selecting
these mitigations to further simplify.

Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
---
 .../admin-guide/kernel-parameters.txt         | 16 +++++++++
 arch/x86/kernel/cpu/bugs.c                    | 34 +++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 09126bb8cc9f..66b567c4dce5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -628,6 +628,21 @@
 	cio_ignore=	[S390]
 			See Documentation/arch/s390/common_io.rst for details.
 
+	clear_cpu_buffers=
+			[X86]
+			Controls the mitigations that use
+			X86_FEATURE_CLEAR_CPU_BUF, namely
+			Micro-architectrual Data Sampling (MDS)
+			MMIO Stale Data
+			TSX Async Abort (TAA)
+			Register File Data Sampling (RFDS)
+
+			The options are:
+			on		- Enable cpu buffer clearing
+			on,nosmt	- Enable cpu buffer clearing and disable
+					  SMT
+			off		- Disables cpu buffer clearing
+
 	clearcpuid=X[,X...] [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit
@@ -3461,6 +3476,7 @@
 				improves system performance, but it may also
 				expose users to several CPU vulnerabilities.
 				Equivalent to: if nokaslr then kpti=0 [ARM64]
+					       clear_cpu_buffers=off [X86]
 					       gather_data_sampling=off [X86]
 					       kvm.nx_huge_pages=off [X86]
 					       l1tf=off [X86]
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 45675da354f3..b3c9e1eede12 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -531,6 +531,40 @@ early_param("reg_file_data_sampling", rfds_parse_cmdline);
 #undef pr_fmt
 #define pr_fmt(fmt)     "" fmt
 
+static int __init clear_cpu_buffers_cmdline(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!boot_cpu_has_bug(X86_BUG_MDS) &&
+	    !boot_cpu_has_bug(X86_BUG_TAA) &&
+	    !boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA) &&
+	    !boot_cpu_has_bug(X86_BUG_RFDS))
+		return 0;
+
+	if (!strcmp(str, "off")) {
+		mds_mitigation = MDS_MITIGATION_OFF;
+		taa_mitigation = TAA_MITIGATION_OFF;
+		mmio_mitigation = MMIO_MITIGATION_OFF;
+		rfds_mitigation = RFDS_MITIGATION_OFF;
+	} else if (!strcmp(str, "on")) {
+		mds_mitigation = MDS_MITIGATION_FULL;
+		taa_mitigation = TAA_MITIGATION_VERW;
+		mmio_mitigation = MMIO_MITIGATION_VERW;
+		rfds_mitigation = RFDS_MITIGATION_VERW;
+	} else if (!strcmp(str, "on,nosmt")) {
+		mds_mitigation = MDS_MITIGATION_FULL;
+		taa_mitigation = TAA_MITIGATION_VERW;
+		mmio_mitigation = MMIO_MITIGATION_VERW;
+		rfds_mitigation = RFDS_MITIGATION_VERW;
+		mds_nosmt = true;
+		taa_nosmt = true;
+		mmio_nosmt = true;
+	}
+	return 0;
+}
+early_param("clear_cpu_buffers", clear_cpu_buffers_cmdline);
+
 static void __init md_clear_update_mitigation(void)
 {
 	if (cpu_mitigations_off())
-- 
2.25.1