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

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Message-ID: <CAAeT=Fw-nt5h3DhRCQr8Ma71NiP7dHB+WD2hie_55SpCHR=mDQ@mail.gmail.com>
Date:   Fri, 30 Apr 2021 12:42:46 -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: [PATCH v4 1/1] x86/cpufeatures: Implement Predictive Store
 Forwarding control.

> +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;
> +               wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +       }
> +
> +       return 0;
> +}

Shouldn't X86_FEATURE_MSR_SPEC_CTRL always be set if the CPU has
X86_FEATURE_PSFD even if the new kernel parameter is not used ?
(e.g. set X86_FEATURE_MSR_SPEC_CTRL in init_speculation_control()
and have psf_cmdline() do the rest)

Considering KVM/virtualization for a CPU that has X86_FEATURE_PSFD
but no other existing feature with MSR_IA32_SPEC_CTRL, if a host
doesn't enable PSFD with the new parameter, the host doesn't have
X86_FEATURE_MSR_SPEC_CTRL.  Then, it would be a problem if its
guests want to use PSFD looking at x86_virt_spec_ctrl().
(I'm not sure how you will change your previous KVM patch though)

Thanks,
Reiji