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Date:      Mon, 18 Feb 2019 18:58:43 -0800
From:      Mark Millard <marklmi@yahoo.com>
To:        Justin Hibbits <chmeeedalf@gmail.com>
Cc:        FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>
Subject:   Question on "Map the entire KVA range into the SLB. We must not fault there" vs. the modern VM_MAX_KERNEL_ADDRESS
Message-ID:  <F456ED61-3CFD-4ACC-84DA-FAEC6270F450@yahoo.com>
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It takes me a bit to provide context for the question . . .

I'll first note:

#define VM_MAX_SAFE_KERNEL_ADDRESS      VM_MAX_KERNEL_ADDRESS

moea64_mid_bootstrap has:

        virtual_avail = VM_MIN_KERNEL_ADDRESS;
        virtual_end = VM_MAX_SAFE_KERNEL_ADDRESS;

        /*
         * Map the entire KVA range into the SLB. We must not fault there.
         */     
        #ifdef __powerpc64__
        for (va = virtual_avail; va < virtual_end; va += SEGMENT_LENGTH)
                moea64_bootstrap_slb_prefault(va, 0);
        #endif

but the prefaulting is based on:

#define PCPU_MD_AIM64_FIELDS                                            \
        struct slb      slb[64];                                        \

where the slb is based on the upper 36 bits of the passed-in va values
(effective address value in powerpc terms).

The address range now being covered is:

#define VM_MIN_KERNEL_ADDRESS           0xe000000000000000
#define VM_MAX_KERNEL_ADDRESS           0xe0000007ffffffff

So for the upper 36 bits:

0xe000_0000_0
0xe000_0007_f

and the range 0x00 through 0x7f has 128 possibilities.

So more than the slb can track. (Also: slb[USER_SLB_SLOT]
is avoided so really 63 of the 64 are available.)

Later (larger) va values are the replacing earlier
(smaller) va values via slb_insert_kernel usage that
in turn involves random replacements via (n_slbs
being 64 in the G5 context):

        i = mftb() % n_slbs;
        if (i == USER_SLB_SLOT)
                        i = (i+1) % n_slbs;

So apparently only the last 63 from:

        for (va = virtual_avail; va < virtual_end; va += SEGMENT_LENGTH)
                moea64_bootstrap_slb_prefault(va, 0);

are known to still be present in the slb.



So the question . . .

How can:

"Map the entire KVA range into the SLB. We must not fault there"

be true? Is it really a requirement? If it is, then something seems
to be wrong as far as handling the modern VM_MAX_KERNEL_ADDRESS value
goes.


The old VM_MAX_KERNEL_ADDRESS value would have 0x00 through 0x1c
and so would fit this particular constraint. The old value
0xe0000001c7ffffff looks like something was being avoided
(why not 0xe0000001ffffffff ?). But I've not found what or why.


===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)

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