Re: [PATCH RFC v9 08/51] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

[Kim Phillips]

On 7/20/23 5:24 PM, Dave Hansen wrote:
> On 7/20/23 12:11, Kim Phillips wrote:
> > Hopefully the commit text in this version will help answer all your
> > questions?:
>
> To be honest, it didn't really.  I kinda feel like I was having the APM
> contents tossed casually in my direction rather than being provided a
> fully considered explanation.

Sorry to hear that, it wasn't the intention.

> Here's what I came up with instead:
>
> Host-side Automatic IBRS has different behavior based on whether 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, nix using Automatic IBRS on SEV-SNP
> hosts.  Fall back to retpolines instead.
>
> =====
>
> Is that about right?

Sure, see new version below.

> I don't think any chit-chat about the guest side is even relevant.
>
> This also absolutely needs a comment.  Perhaps just pull the code up to
> the top level of the function and do this:
>
> 	/*
> 	 * Automatic IBRS imposes unacceptable overhead on host
> 	 * userspace for SEV-SNP systems.  Zap it instead.
> 	 */
> 	if (cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> 		setup_clear_cpu_cap(X86_FEATURE_AUTOIBRS);

Clearing X86_FEATURE_AUTOIBRS won't work because it'll unnecessarily
prohibit KVM from subsequently advertising the feature to guests.

I've tried to address the comment comment, see below.

> BTW, I assume you've grumbled to folks about this.  It's an awful shame
> the hardware (or ucode) was built this was.  It's just throwing
> Automatic IBRS out the window because it's not architected in a nice way.
>
> Is there any plan to improve this?

Sure.  Until then:

From fb55df544ed066a3c8fdb1581932a23c03ce6d2c Mon Sep 17 00:00:00 2001
From: Kim Phillips <kim.phillips@xxxxxxx>
Date: Mon, 17 Jul 2023 14:08:15 -0500
Subject: [PATCH] x86/speculation: Don't enable Automatic IBRS if SEV SNP is
 enabled

Host-side Automatic IBRS has a different behaviour depending on whether
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.  Fall back to retpolines instead.

Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
---
 arch/x86/kernel/cpu/common.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8cd4126d8253..a93e6204d847 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1347,8 +1347,11 @@ 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))
--
2.34.1