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Subject: Re: [PATCH v4 1/1] x86/cpufeatures: Implement Predictive Store
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To:     Reiji Watanabe <reijiw@google.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        Jonathan Corbet <corbet@lwn.net>, bsd@redhat.com
References: <20210430131733.192414-1-rsaripal@amd.com>
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From:   "Saripalli, RK" <rsaripal@amd.com>
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Date:   Fri, 30 Apr 2021 14:52:08 -0500
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On 4/30/2021 2:42 PM, Reiji Watanabe wrote:
>> +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)

Reiji, you are correct that X86_FEATURE_MSR_SPEC_CTRL should be enabled so KVM guests can use PSFD
even if host does not use it.
I have this change in my KVM patch.

Thanks for the review,
RK
> 
> Thanks,
> Reiji
>