linux-kernel - Re: [PATCH v4] cper, apei, mce: Pass x86 CPER through the MCA handling chain

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Message-ID: <877dsiislt.fsf@kokedama.swc.toshiba.co.jp>
Date:   Fri, 25 Sep 2020 09:54:06 +0900
From:   Punit Agrawal <punit1.agrawal@...hiba.co.jp>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Smita Koralahalli Channabasappa <skoralah@....com>,
        Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-acpi@...r.kernel.org,
        devel@...ica.org, Tony Luck <tony.luck@...el.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Yazen Ghannam <yazen.ghannam@....com>
Subject: Re: [PATCH v4] cper, apei, mce: Pass x86 CPER through the MCA handling chain

Borislav Petkov <bp@...en8.de> writes:

> On Thu, Sep 24, 2020 at 12:23:27PM -0500, Smita Koralahalli Channabasappa wrote:
>> > Even though it's not defined in the UEFI spec, it doesn't mean a
>> > structure definition cannot be created.
>
> Created for what? That structure better have a big fat comment above it, what
> firmware generates its layout.

Maybe I could've used a better choice of words - I meant to define a
structure with meaningful member names to replace the *(ptr + i)
accesses in the patch.

The requirement for documenting the record layout doesn't change -
whether using raw pointer arithmetic vs a structure definition.

>> > After all, the patch is relying on some guarantee of the meaning of
>> > the values and their ordering.
>
> AFAICT, this looks like an ad-hoc definition and the moment they change
> it in some future revision, that struct of yours becomes invalid so we'd
> need to add another one.

If there's no spec backing the current layout, then it'll indeed be an
ad-hoc definition of a structure in the kernel. But considering that
it's part of firmware / OS interface for an important part of the RAS
story I would hope that the code is based on a spec - having that
reference included would help maintainability.

Incompatible changes will indeed break the assumptions in the kernel and
code will need to be updated - regardless of the choice of kernel
implementation; pointer arithmetic, structure definition - ad-hoc or
spec provided.

Having versioning will allow running older kernels on newer hardware and
vice versa - but I don't see why that is important only when using a
structure based access.

>
>> > If the patch is relying on the definitions in the SMCA spec it is a good
>
> Yes, what SMCA spec is that?
>
>> > idea to reference it here - both for review and providing relevant
>> > context for future developers.
>> 
>> Okay, I agree the structure definition will make the code less arbitrary
>> and provides relevant context compared to pointer arithmetic. I did not
>> think this way. I can try this out if no objections.
>
> Again, this struct better have "versioning" info because the moment your
> fw people change it in some future platform, this code needs touching
> again.
>
> It probably would need touching even with the offsets if those offsets
> change but at least not having it adhere to some slow-moving spec is
> probably easier in case they wanna add/change fields.
>
> So Smita, you probably should talk to fw people about how stable that
> layout at ctx_info + 1 is going to be wrt future platforms so that
> we make sure we only access the correct offsets, now and on future
> platforms.
>
> Thx.