linux-kernel - Re: [PATCH v4 1/1] x86/cpufeatures: Implement Predictive Store Forwarding control.

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Date:   Tue, 4 May 2021 17:11:54 -0700
From:   Pawan Gupta <writetopawan@...il.com>
To:     Ramakrishna Saripalli <rsaripal@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        Jonathan Corbet <corbet@....net>, bsd@...hat.com
Subject: Re: [PATCH v4 1/1] x86/cpufeatures: Implement Predictive Store
 Forwarding control.

On 30.04.2021 08:17, Ramakrishna Saripalli wrote:
>--- a/arch/x86/kernel/cpu/amd.c
>+++ b/arch/x86/kernel/cpu/amd.c
>@@ -1170,3 +1170,26 @@ void set_dr_addr_mask(unsigned long mask, int dr)
> 		break;
> 	}
> }
>+
>+static int __init psf_cmdline(char *str)
>+{
>+	u64 tmp = 0;
>+
>+	if (!boot_cpu_has(X86_FEATURE_PSFD))
>+		return 0;
>+
>+	if (!str)
>+		return -EINVAL;
>+
>+	if (!strcmp(str, "off")) {
>+		set_cpu_cap(&boot_cpu_data, X86_FEATURE_MSR_SPEC_CTRL);
>+		rdmsrl(MSR_IA32_SPEC_CTRL, tmp);
>+		tmp |= SPEC_CTRL_PSFD;
>+		x86_spec_ctrl_base |= tmp;

I don't think there is a need to update x86_spec_ctrl_base here.
check_bugs() already reads the MSR_IA32_SPEC_CTRL and updates
x86_spec_ctrl_base. 

>+		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
>+	}
>+
>+	return 0;
>+}
>+
>+early_param("predict_store_fwd", psf_cmdline);
>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;

This change also doesn't seem to be necessary, psf_cmdline() updates the
MSR( i.e. sets PSFD).  Here read from the MSR will still update
x86_spec_ctrl_base to the correct value. Am I missing something?

Thanks,
Pawan