linux-kernel - Re: [PATCH v2 1/8] x86/CPU/AMD: Save NodeId on AMD-based systems

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Message-ID: <20200917194425.GB504453@yaz-nikka.amd.com>
Date:   Thu, 17 Sep 2020 14:44:25 -0500
From:   Yazen Ghannam <yazen.ghannam@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        tony.luck@...el.com, x86@...nel.org,
        Smita.KoralahalliChannabasappa@....com
Subject: Re: [PATCH v2 1/8] x86/CPU/AMD: Save NodeId on AMD-based systems

On Thu, Sep 17, 2020 at 06:40:48PM +0200, Borislav Petkov wrote:
> On Thu, Sep 17, 2020 at 11:20:53AM -0500, Yazen Ghannam wrote:
> > But newer systems support CPUID Leaf 0xB, so cpu_die_id will get
> > explicitly set by detect_extended_topology(). The value set is
> > different from the AMD NodeId. And at that point I shied away from
> > doing any override or fixup.
> 
> Well, different how? Can you extract the node_id you need
> from CPUID(0xb)? If yes, we can do an AMD-specific branch in
> detect_extended_topology() but that better be future proof.
> 
> IOW, is information from CPUID(0xb) ever going to be needed in the
> kernel?
> 
> Also, and independently, if its definition do not give you the
> node_id you need, then you can just as well overwrite ->cpu_die_id in
> detect_extended_topology() because that value - whatever that is, could
> be garbage, just as well - is wrong on AMD anyway.
> 
> So it would be a fix for the leaf parsing, regardless of whether you
> need it or not.
> 
> Makes sense?
>

Yes, I think so. "Die" is not defined in CPUID(0xb), only SMT and Core,
so the cpu_die_id value is not valid. In which case, we can overwrite
it. CPUID(0xb) doesn't have anything equivalent to AMD NodeId. So on
systems with CPUID < 0x1F, we should be okay with using cpu_die_id equal
to AMD NodeId.

I have an idea on what to do, so I'll send another rev if that's okay.
Do you have any comments on the other patches in the set?

Thanks,
Yazen