hi,

i've been looking lately at the different mmmmap() implementations
each of our archs has and noticed some inconsistency.

first, each mmmmap() routine can be (usually) found in
src/sys/arch/<arch>/<arch>/mem.c:mmmmap()

we'll take a look at differences between several archs: i386, amd64,
alpha, vax, hp300, powerpc, and sun3. for all archs, i'll be referring
to the access conditions for DEV_MEM.

i386:
if (check_pa_acc(off, prot) != 0) {
	return -1;
}

(and check_pa_acc() is:)

static int
check_pa_acc(paddr_t pa, vm_prot_t prot)
{
	extern phys_ram_seg_t mem_clusters[VM_PHYSSEG_MAX];
	extern int mem_cluster_cnt;
	int i;

	if (securelevel <= 0) {
		return 0;
	}

	for (i = 0; i < mem_cluster_cnt; i++) {
		const phys_ram_seg_t *seg = &mem_clusters[i];
		paddr_t start = seg->start;

		if (start <= pa && pa - start <= seg->size) {
			return 0;
		}
	}

	return EPERM;
}

as we can see above, freely accessing physical memory, even outside
ram, is only permitted in securelevel 0 and below.

this is very similar to what happens on alpha:

if ((prot & alpha_pa_access(off)) != prot)
	return (-1);

(and alpha_pa_access() is:)

int
alpha_pa_access(pa)
	u_long pa;
{
	int i;

	for (i = 0; i < mem_cluster_cnt; i++) {
		if (pa < mem_clusters[i].start)
			continue;
		if ((pa - mem_clusters[i].start) >=
		    (mem_clusters[i].size & ~PAGE_MASK))
			continue;
		return (mem_clusters[i].size & PAGE_MASK); /* prot */
	}

	if (securelevel > 0)
		return (PROT_NONE);
	else
		return (PROT_READ | PROT_WRITE);
}

so far so good. however, looking at amd64, vax, and powerpc we see
different behavior:

amd64:
if (off > ctob(physmem) && kauth_authorize_generic(l->l_cred,
    KAUTH_GENERIC_ISSUSER, &l->l_acflag) != 0)
		return (-1);

vax:
if ((u_int)off > ctob(physmem) && kauth_authorize_generic(l->l_cred,
    KAUTH_GENERIC_ISSUSER, &l->l_acflag) != 0)
		return (-1);

powerpc:
if (atop(off) >= physmem && kauth_authorize_generic(l->l_cred,
    KAUTH_GENERIC_ISSUSER, &l->l_acflag) != 0)
		return (-1);

but alas, looking further, for example, hp300 and sun3, we see a
different behavior again:

hp300:
if ((u_int)off < lowram || (u_int)off >= 0xFFFFFFFC)
	return -1;

sun3:
if (off < avail_start || off >= avail_end)
	break;


now; i'm not an expert on any of these architectures so i can't tell
if there are special reasons why *eventually* these somehow achieve the
same behavior via different code, but the way i see it, we have three
(or more) kinds of behavior for more-or-less access to the same type
of resource (physical memory) on different architectures.

what i'm asking, is if the above is intentional (and if yes, if there
is any documentation on the matter), and if it's not, what should be
the consistent behavior.

thanks,

-e.

-- 
Elad Efrat