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

[Guenter Roeck]

On 9/12/21 12:58 PM, Helge Deller wrote:
> * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>:
> > On Sun, Sep 12, 2021 at 9:02 AM Guenter Roeck <linux@xxxxxxxxxxxx> 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).
>
> Yes, this second approach seems to work nicely, although the patch
> then gets slightly bigger.
> Below is a tested patch.
> I'll check it some further and apply it to the parisc tree then.

There are several PAGE0-> references left in the code after applying your patch.

$ git grep "PAGE0->"
arch/parisc/kernel/firmware.c:  if (!PAGE0->mem_kbd.iodc_io)
arch/parisc/kernel/firmware.c:  real32_call(PAGE0->mem_kbd.iodc_io,
arch/parisc/kernel/firmware.c:              (unsigned long)PAGE0->mem_kbd.hpa, ENTRY_IO_CIN,
arch/parisc/kernel/firmware.c:              PAGE0->mem_kbd.spa, __pa(PAGE0->mem_kbd.dp.layers),
arch/parisc/kernel/smp.c:       WARN_ON(((unsigned long)(PAGE0->mem_pdc_hi) << 32
arch/parisc/kernel/smp.c:                       | PAGE0->mem_pdc) != pdce_proc);

After fixing those, I can build a parisc image and boot it in qemu (32 bit).

Guenter