linux-kernel - Re: [PATCH 1/5] x86/mce/inject: Check if a bank is unpopulated before error simulation

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Date:   Thu, 14 Oct 2021 15:26:13 -0500
From:   "Koralahalli Channabasappa, Smita" <skoralah@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
        x86@...nel.org, linux-edac@...r.kernel.org,
        linux-kernel@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, yazen.ghannam@....com
Subject: Re: [PATCH 1/5] x86/mce/inject: Check if a bank is unpopulated before
 error simulation

On 10/14/21 1:22 PM, Borislav Petkov wrote:

> On Mon, Oct 11, 2021 at 04:12:14PM -0500, Koralahalli Channabasappa, Smita wrote:
>> I do not have the bank number in order to look up the IPID for that bank.
>> I couldn't know the bank number because mce-inject files are synchronized
>> in a way that once the bank number is written the injection starts.
>> Can you please suggest what needs to be done here?
>>
>> Also, the IPID register is read only from the OS, hence the user provided
>> IPID values could be useful for "sw" error injection types. For "hw" error
>> injection types we need to read from the registers to determine the IPID
>> value.
>>
>> Should there be two cases where on a "sw" injection use the user provided
>> IPID value whereas on "hw" injection read from registers?
> Right, that's a good point. So the way I see it is, we need to decide
> what is allowed for sw injection and what for hw injection, wrt to IPID
> value.
>
> I think for sw injection, we probably should say that since this is
> sw only and its purpose is to test the code only, there should not be
> any limitations imposed by the underlying machine. Like using the bank
> number, for example.
>
> So what you do now for sw injection:
>
> 		if (val && inj_type == SW_INJ)
> 			m->ipid = val;
>
> should be good enough. User simply sets some IPID value and that value
> will be used for the bank which is written when injecting.
>
> Now, for hw injection, you have two cases:
>
> 1. The bank is unpopulated so setting the IPID there doesn't make any sense.
>
> 2. The bank *is* populated and the respective IPID MSR has a value
> describing what that bank is.
>
> And in that case, does it even make sense to set the IPID? I don't think
> so because that IP block's type - aka IPID - has been set already by
> hardware/firmware.
>
> So the way I see it, it makes no sense whatsoever to set the IPID of a
> bank during hw injection.
>
> Right?

Yes, I agree. inj_ipid_set() can be used to serve the purpose of setting
user provided IPID on a sw injection only.

My concern was, we need to determine whether the bank is unpopulated or
populated before trying to inject the errors on a hw injection, for which
we need to read the IPID MSR of that bank.

We cannot do that inside inj_ipid_set() as we do not know the bank number
until inj_bank_set() executes which is called after inj_ipid_set().
mce-inject files are synchronized in a way that once the bank number is
written in inj_bank_set(), injection starts.

So this snippet of code:

if (inj_type != SW_INJ) {
	rdmsrl_on_cpu(m->extcpu, MSR_AMD64_SMCA_MCx_IPID(val),&m->ipid);
	if (!m->ipid) {
		pr_err("Error injection not possible - bank %d unpopulated\n");
		return -ENODEV;
	}
}

should be retained inside inj_bank_set() ?

And inj_ipid_set() should just set m->ipid = val on a SW_INJ as you mentioned
above?

Thanks,