linux-kernel - Re: [RFC][PATCH 05/17] x86/cpu: Cleanup the untrain mess

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Date:   Sat, 12 Aug 2023 20:30:09 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, gregkh@...uxfoundation.org
Subject: Re: [RFC][PATCH 05/17] x86/cpu: Cleanup the untrain mess

On Wed, Aug 09, 2023 at 03:26:35PM +0200, Peter Zijlstra wrote:
> I think simply setting UNRET for SRSO at this point will be sufficient.
> That ensures the entry_untrain_ret thing gets called, and the
> alternative there DTRT.
> 
> The feature isn't used anywhere else afaict.
> 
> Then later, after the fancy alternatives happen, this can be cleaned up
> again.

Yes, this fixes it for >= Zen3:

---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4b0a770fbacb..611d048f6415 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2448,6 +2448,7 @@ static void __init srso_select_mitigation(void)
 			 * like ftrace, static_call, etc.
 			 */
 			setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+			setup_force_cpu_cap(X86_FEATURE_UNRET);
 
 			if (boot_cpu_data.x86 == 0x19) {
 				setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);

-- 
Regards/Gruss,
    Boris.

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