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

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Date:   Thu, 29 Apr 2021 09:01:09 -0500
From:   "Saripalli, RK" <rsaripal@....com>
To:     Reiji Watanabe <reijiw@...gle.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: [v3 1/1] x86/cpufeatures: Implement Predictive Store Forwarding
 control.



On 4/29/2021 12:38 AM, Reiji Watanabe wrote:
>> +       if (!strcmp(str, "off")) {
>> +               set_cpu_cap(&boot_cpu_data, X86_FEATURE_MSR_SPEC_CTRL);
>> +               x86_spec_ctrl_base |= SPEC_CTRL_PSFD;
>> +               wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> 
> My previous suggestion about updating MSR_IA32_SPEC_CTRL meant
> something like:
> 
>     rdmsrl(MSR_IA32_SPEC_CTRL, current);
>     wrmsrl(MSR_IA32_SPEC_CTRL, current | SPEC_CTRL_PSFD);
> 
> And this is to keep the behavior of code below check_bugs().
> (Or do you intentionally change it due to some reason ?)
> BTW, x86_spec_ctrl_base, which is updated in psf_cmdline(),
> would be overwritten by check_bugs() anyway as follows.
> ---

Since psf_cmdline() directly writes to the MSR itself (and it only does this)
if the feature is available (per CPUID), check_bugs() should be ok.

My patch for now does not depend on the value of x86_spec_ctrl_base after psf_cmdline()
finishes execution.

> void __init check_bugs(void)
> {
>         <...>
>         /*
>          * Read the SPEC_CTRL MSR to account for reserved bits which may
>          * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
>          * 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);
>         <...>
> ---
> 
>> +               setup_clear_cpu_cap(X86_FEATURE_PSFD);
> 
> Does X86_FEATURE_PSFD need to be cleared for the 'off' case ?
> Do you want to remove "psfd" from /proc/cpuinfo
> when PSFD is enabled ? (not when PSFD is disabled ?)
> 
> 
No, it should not be cleared, I agree.
But I did test with KVM (with my patch that is not here) and I do not see
issues (meaning user space guest in QEMU is seeing PSF CPUID guest capability)

I see no reason to clear this feature and I will submit a new patch with this and other changes.

> Thanks,
> Reiji
>