Commit acaa4b5c authored by Kim Phillips's avatar Kim Phillips Committed by Borislav Petkov (AMD)
Browse files

x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled 



Without SEV-SNP, Automatic IBRS protects only the kernel. But when
SEV-SNP is enabled, the Automatic IBRS protection umbrella widens to all
host-side code, including userspace. This protection comes at a cost:
reduced userspace indirect branch performance.

To avoid this performance loss, don't use Automatic IBRS on SEV-SNP
hosts and all back to retpolines instead.

  [ mdr: squash in changes from review discussion. ]

Signed-off-by: default avatarKim Phillips <kim.phillips@amd.com>
Signed-off-by: default avatarMichael Roth <michael.roth@amd.com>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Acked-by: default avatarDave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/r/20240126041126.1927228-3-michael.roth@amd.com
parent b6e0f666

arch/x86/kernel/cpu/common.c

+6 −1
Original line number Diff line number Diff line
@@ -1355,8 +1355,13 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c) 
	/*

	 * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel feature

	 * flag and protect from vendor-specific bugs via the whitelist.

	 *

	 * Don't use AutoIBRS when SNP is enabled because it degrades host

	 * userspace indirect branch performance.

	 */

	if ((ia32_cap & ARCH_CAP_IBRS_ALL) || cpu_has(c, X86_FEATURE_AUTOIBRS)) {

	if ((ia32_cap & ARCH_CAP_IBRS_ALL) ||

	    (cpu_has(c, X86_FEATURE_AUTOIBRS) &&

	     !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {

		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);

		if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&

		    !(ia32_cap & ARCH_CAP_PBRSB_NO))