linux-kernel - Re: [v2 1/1] x86/cpufeatures: Implement Predictive Store Forwarding control.

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Date:   Mon, 26 Apr 2021 09:28:40 -0700
From:   Reiji Watanabe <reijiw@...gle.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: [v2 1/1] x86/cpufeatures: Implement Predictive Store Forwarding control.

> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1170,3 +1170,22 @@ void set_dr_addr_mask(unsigned long mask, int dr)
>                 break;
>         }
>  }
> +
> +static int __init psf_cmdline(char *str)
> +{
> +       if (!boot_cpu_has(X86_FEATURE_PSFD))
> +               return 0;
> +
> +       if (!str)
> +               return -EINVAL;
> +
> +       if (!strcmp(str, "off")) {
> +               x86_spec_ctrl_base |= SPEC_CTRL_PSFD;
> +               wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +               setup_clear_cpu_cap(X86_FEATURE_PSFD);

Shouldn't X86_FEATURE_MSR_SPEC_CTRL be set if the CPU supports PSF ?
x86_spec_ctrl_setup_ap(), which is called on non-boot CPUs, doesn't
update MSR_IA32_SPEC_CTRL with x86_spec_ctrl_base not having
X86_FEATURE_MSR_SPEC_CTRL (i.e. if a CPU supports PSF but no other
existing feature that makes the kernel set X86_FEATURE_MSR_SPEC_CTRL).

Also, since check_bugs() reads the SPEC_CTRL MSR to account for reserved
bits which may have unknown bits to set x86_spec_ctrl_base
(if X86_FEATURE_MSR_SPEC_CTRL is set),
I'm wondering if psf_cmdline(), which is called earlier
than check_bugs(), should do the same instead of overwriting
it with x86_spec_ctrl_base | SPEC_CTRL_PSFD.

Thanks,
Reiji