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Re: [Qemu-devel] [RFC][PATCH 10/15] memory: Rework sub-page handling

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC][PATCH 10/15] memory: Rework sub-page handling
Date: Tue, 07 May 2013 14:35:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4

Il 06/05/2013 22:46, Peter Maydell ha scritto:
> On 6 May 2013 15:26, Jan Kiszka <address@hidden> wrote:
>> Simplify the sub-page handling by implementing it directly in the
>> dispatcher instead of using a redirection memory region. We extend the
>> phys_sections entries to optionally hold a pointer to the sub-section
>> table that used to reside in the subpage_t structure. IOW, we add one
>> optional dispatch level below the existing radix tree.
>> address_space_lookup_region is extended to take this additional level
>> into account. This direct dispatching to that target memory region will
>> also be helpful when we want to add per-region locking control.
> This patch seems to break vexpress-a9. Test case if you want it:
> http://staging.people.linaro.org/~peter.maydell/vexpress-3.8.tar.gz
> (125MB) Edit the 'runme' script to fix up the paths to kernel/initrd/dtb
> and then run it; before this patch it boots, afterwards it doesn't
> even manage to start the kernel.
> My guess is you've broken subregion-sized mmio regions somehow
> (and/or regions which are larger than a page in size but start
> or finish at a non-page-aligned address), and probably in particular
> the arm_gic regions that a9mpcore maps...

I think we just found out what all the subpage stuff is for. :)

When I added address_space_translate(), the trickiest conversion to the
new API was in tlb_set_page.  Hence I added a "you never know"-style assert:

    assert(size >= TARGET_PAGE_SIZE);
    if (size != TARGET_PAGE_SIZE) {
        tlb_add_large_page(env, vaddr, size);
-    section = phys_page_find(address_space_memory.dispatch,
-                            paddr >> TARGET_PAGE_BITS);
+    sz = size;
+    section = address_space_translate(&address_space_memory, paddr,
+                                     &xlat, &sz, false);
+    assert(sz >= TARGET_PAGE_SIZE);

Now, remember that address_space_translate clamps sz on output to the
size of the section.  And here's what happens:

(gdb) p sz
$4 = 256
(gdb) p *(section->mr)
$5 = {ops = 0x7ffff82d33c0, iommu_ops = 0x0, opaque = 0x7ffff91e6b50,
parent = 0x7ffff9069400, owner = 0x0, size = {lo = 256,
    hi = 0}, addr = 0, destructor = 0x7ffff7e824d0
<memory_region_destructor_none>, ram_addr = 18446744073709551615,
terminates =
    true, romd_mode = true, ram = false, readonly = false, enabled =
true, rom_device = false, warning_printed = false,
  flush_coalesced_mmio = false, alias = 0x0, alias_offset = 0, priority
= 0, may_overlap = false, subregions = {tqh_first = 0x0,
    tqh_last = 0x7ffff91e8ee8}, subregions_link = {tqe_next = 0x0,
tqe_prev = 0x7ffff91e64f8}, coalesced = {tqh_first = 0x0,
    tqh_last = 0x7ffff91e8f08}, name = 0x7ffff906bb60 "a9-scu",
dirty_log_mask = 0 '\000', ioeventfd_nb = 0, ioeventfds = 0x0,
  iommu_target_as = 0x0}

The TLB would only store this region instead of the whole page, and
subsequent access past the first 256 bytes would be emulated incorrectly.

For the record, I attach the patch I was using to fix Jan's.


Attachment: 0001-subpage-fix.patch
Description: Text Data

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