From: Kim Phillips <kim.phillips@amd.com>

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. Fall back to retpolines instead.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
[mdr: squash in changes from review discussion]
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 arch/x86/kernel/cpu/common.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 382d4e6b848d..11fae89b799e 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1357,8 +1357,13 @@ 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))
-- 
2.25.1

On 10/16/23 15:27, Michael Roth wrote:
> From: Kim Phillips <kim.phillips@amd.com>
> 
> 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. Fall back to retpolines instead.
> 
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>
> [mdr: squash in changes from review discussion]
> Signed-off-by: Michael Roth <michael.roth@amd.com>

Queued, thanks.

Paolo

> ---
>   arch/x86/kernel/cpu/common.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 382d4e6b848d..11fae89b799e 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1357,8 +1357,13 @@ 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))

On 10/16/23 06:27, Michael Roth wrote:
> 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. Fall back to retpolines instead.

Thanks for the updated changelog:

Acked-by: Dave Hansen <dave.hansen@intel.com>

BTW, have you given your hardware folks a hard time about this?  It
seems _kinda_ silly to be using retpolines when the hardware has a
perfectly good IBRS implementation for the kernel.

Just please make sure there's a good underlying reason for this behavior
and as opposed to being some kind of inadvertent side effect.

I assume Auto-IBRS and SEV-SNP are going to be with us for a long time,
so it would be nice to have a long term solution here.

On Mon, Oct 16, 2023 at 08:27:34AM -0500, Michael Roth wrote:
> From: Kim Phillips <kim.phillips@amd.com>
> 
> 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. Fall back to retpolines instead.
> 
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>
> [mdr: squash in changes from review discussion]
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>  arch/x86/kernel/cpu/common.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Acked-by: Borislav Petkov (AMD) <bp@alien8.de>

-- 
Regards/Gruss,
    Boris.

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