From owner-freebsd-arch  Wed Oct  3  4:48:52 2001
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Date: Wed, 3 Oct 2001 21:47:49 +1000 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Mike Silbersack <silby@silby.com>
Cc: Dan Nelson <dnelson@allantgroup.com>, <freebsd-arch@FreeBSD.ORG>
Subject: Re: Reading physical memory in a cross-platform way
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On Tue, 2 Oct 2001, Mike Silbersack wrote:

> On Mon, 1 Oct 2001, Dan Nelson wrote:
>
> > > physmem != hw.physmem.  The sysctl is actually the output of a function,
> > > not a simple int or the like.
> >
> > Yeah, but look at the function (i386 as an example):
> >
> > static int
> > sysctl_hw_physmem(SYSCTL_HANDLER_ARGS)
> > {
> >     int error = sysctl_handle_int(oidp, 0, ctob(physmem), req);
> >     return (error);
> > }
> >
> > So the physmem value is just in pages; even though the different
> > platforms use ctob() or <mach>_ptob(), the macros all expand to
> > ((x) << PAGE_SHIFT).

Using ptob() (pages to bytes) would be correct, except ptob() doesn't
exist.  There are "only" <mach>_ptob(), ptoa() (pages to address) and
the anachronism ctob() (clicks to bytes), not to mention explicit shifts
and multiplications, to do this conversion.

> True, but I still don't want to spread MD-ish code across the kernel.  So,
> I whipped up the patch I proposed.  This introduces "maxmembytes", which
> contains the size of ram in bytes.  I think I've updated all architectures
> properly, but I've only tested on i386.  Could someone working on
> alpha/ia64/sparc check to make sure that it doesn't break compilation?
>
> Unless there are objections, I'll probably commit it in a few days.

No thanks.  This spreads MI-ish code across all arches.

Note that physmem is already used in MI code, in vfs_bio_alloc() and
vm_ksubmap_init().  vfs_bio_alloc() is not actually MI -- machine-
dependencies in this code include the magic number 16384 (64MB in
i386 pages).  vm_ksubmap_init() uses btoc() (bytes to clicks) to
do the reverse conversion to the one you want.

Bruce


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