linux-kernel - Re: [v3 1/1] x86/cpufeatures: Implement Predictive Store Forwarding control.

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Date:   Thu, 29 Apr 2021 17:02:42 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Saripalli, RK" <rsaripal@....com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Reiji Watanabe <reijiw@...gle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, Jonathan Corbet <corbet@....net>,
        bsd@...hat.com
Subject: Re: [v3 1/1] x86/cpufeatures: Implement Predictive Store Forwarding
 control.

On Thu, Apr 29, 2021 at 09:32:35AM -0500, Saripalli, RK wrote:
> Yes, I agree with his analysis and fixing it.

So you can do this and correct the comment above it to explain why
you're doing the "tmp" thing.

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d41b70fe4918..536136e0daa3 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -78,6 +78,8 @@ EXPORT_SYMBOL_GPL(mds_idle_clear);
 
 void __init check_bugs(void)
 {
+	u64 tmp = 0;
+
 	identify_boot_cpu();
 
 	/*
@@ -97,7 +99,9 @@ void __init check_bugs(void)
 	 * init code as it is not enumerated and depends on the family.
 	 */
 	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
-		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		rdmsrl(MSR_IA32_SPEC_CTRL, tmp);
+
+	x86_spec_ctrl_base |= tmp;
 
 	/* Allow STIBP in MSR_SPEC_CTRL if supported */
 	if (boot_cpu_has(X86_FEATURE_STIBP))

---

and as Tom correctly suggests, set X86_FEATURE_MSR_SPEC_CTRL in
psf_cmdline() so that the above loading of the base value works.

Thx.

-- 
Regards/Gruss,
    Boris.

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