linux-kernel - Re: [PATCH 3/4] parisc: Use absolute_pointer for memcmp on fixed memory location

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Message-ID: <CAHk-=wi1TBvyk7SWX+5LLYN8ZnTJMut1keQbOrKCG=nb08hdiQ@mail.gmail.com>
Date:   Sun, 12 Sep 2021 12:11:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        alpha <linux-alpha@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-parisc@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: Re: [PATCH 3/4] parisc: Use absolute_pointer for memcmp on fixed
 memory location

On Sun, Sep 12, 2021 at 9:02 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> -       running_on_qemu = (memcmp(&PAGE0->pad0, "SeaBIOS", 8) == 0);
> +       running_on_qemu = (memcmp(absolute_pointer(&PAGE0->pad0), "SeaBIOS", 8) == 0);

This seems entirely the wrong thing to do, and makes no sense. That
"&PAGE0->pad0" is a perfectly valid pointer, and that's not where the
problem is.

The problem is "PAGE0" itself:

    #define PAGE0   ((struct zeropage *)__PAGE_OFFSET)

which takes that absolute offset and creates a pointer out of it.

IOW, _that_ is what should have the "absolute_pointer()" thing, and in
that context the name of that macro and its use actually makes sense.

No?

An alternative - and possibly cleaner - approach that doesn't need
absolute_pointer() at all might be to just do

        extern struct zeropage PAGE0;

and then make that PAGE0 be defined to __PAGE_OFFSET in the parisc
vmlinux.lds.S file.

Then doing things like

        running_on_qemu = !memcmp(&PAGE0.pad0, "SeaBIOS", 8);

would JustWork(tm).

Hmm?

             Linus