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

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Date:   Mon, 26 Apr 2021 11:40:40 -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: [v2 1/1] x86/cpufeatures: Implement Predictive Store Forwarding
 control.



On 4/26/2021 11:28 AM, Reiji Watanabe wrote:
>> --- 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).

Reiji, that is a good catch. 
This patch unconditionally sets to bit 7 even on machines where bit 7 is undefined thereby risking a kernel oops.
I will fix this.

I did test this code on a Milan machine and verified that with the setting to off, the SPEC_CTRL MSR was showing bit 7 set to 1 and with on setting SPEC_CTRL MSR bit 7 was cleared.
I tested this with both spec_ctrl_bypass_disable kernel parameter and without. 
I verified with {sudo modprobe msr; rdmsr -a 72 which dumps on all CPUs.
But I may not have tested this patch on a non-Milan machine with both settings.
I will make sure to test it on non-Milan machines before sending the next version out.

> 
> 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.
> 
Ok. I did not think of that path. I will go through that code and fix up the patch.
> 
> Thanks,
> Reiji
> 

Thanks,
RK