[Xen-devel] c/s 22402 ("86 hvm: Refuse to perform __hvm_copy() work in atomic context.") breaks HVM, race possible in other code - any ideas?

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Jan 11, 2011 at 09:52:19AM -0500, Daniel De Graaf wrote:
> On 01/11/2011 08:15 AM, Daniel De Graaf wrote:
> > On 01/10/2011 05:41 PM, Konrad Rzeszutek Wilk wrote:
> >>> @@ -284,8 +304,25 @@ static void unmap_grant_pages(struct grant_map *map, 
> >>> int offset, int pages)
> >>>           goto out;
> >>>  
> >>>   for (i = 0; i < pages; i++) {
> >>> +         uint32_t check, *tmp;
> >>>           WARN_ON(unmap_ops[i].status);
> >>> -         __free_page(map->pages[offset+i]);
> >>> +         if (!map->pages[i])
> >>> +                 continue;
> >>> +         /* XXX When unmapping, Xen will sometimes end up mapping the GFN
> >>> +          * to an invalid MFN. In this case, writes will be discarded and
> >>> +          * reads will return all 0xFF bytes. Leak these unusable GFNs
> >>> +          * until a way to restore them is found.
> >>> +          */
> >>> +         tmp = kmap(map->pages[i]);
> >>> +         tmp[0] = 0xdeaddead;
> >>> +         mb();
> >>> +         check = tmp[0];
> >>> +         kunmap(map->pages[i]);
> >>> +         if (check == 0xdeaddead)
> >>> +                 __free_page(map->pages[i]);
> >>> +         else if (debug)
> >>> +                 printk("%s: Discard page %d=%ld\n", __func__,
> >>> +                         i, page_to_pfn(map->pages[i]));
> >>
> >> Whoa. Any leads to when the "sometimes" happens? Does the status report an
> >> error or is it silent?
> > 
> > Status is silent in this case. I can produce it quite reliably on my
> > test system where I am mapping a framebuffer (1280 pages) between two
> > HVM guests - in this case, about 2/3 of the released pages will end up
> > being invalid. It doesn't seem to be size-related as I have also seen
> > it on the small 3-page page index mapping. There is a message on xm
> > dmesg that may be related:
> > 
> > (XEN) sh error: sh_remove_all_mappings(): can't find all mappings of mfn 
> > 7cbc6: c=8000000000000004 t=7400000000000002
> > 
> > This appears about once per page, with different MFNs but the same c/t.
> > One of the two HVM guests (the one doing the mapping) has the PCI
> > graphics card forwarded to it.
> > 
> 
> Just tested on the latest xen 4.1 (with 22402:7d2fdc083c9c reverted as
> that breaks HVM grants), which produces different output:

Keir, the c/s 22402 has your name on it.

Any ideas on the problem that Daniel is hitting with unmapping grants?
> 
> ...
> (XEN) mm.c:889:d1 Error getting mfn b803e (pfn 25a3e) from L1 entry 
> 00000000b803e021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn b8038 (pfn 25a38) from L1 entry 
> 00000000b8038021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn b803d (pfn 25a3d) from L1 entry 
> 00000000b803d021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn 10829 (pfn 25a29) from L1 entry 
> 0000000010829021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn 1081c (pfn 25a1c) from L1 entry 
> 000000001081c021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn 10816 (pfn 25a16) from L1 entry 
> 0000000010816021 for l1e_owner=1, pg_owner=1
> (XEN) mm.c:889:d1 Error getting mfn 1081a (pfn 25a1a) from L1 entry 
> 000000001081a021 for l1e_owner=1, pg_owner=1
> ...
> 
> This appears on the map; nothing is printed on the unmap. If the
> unmap happens while the domain is up, it seems to be invalid more often;
> most (perhaps all) of the destination-valid unmaps happen when the domain
> is being destroyed. Exactly which pages are valid or invalid seems to be
> mostly random, although nearby GFNs tend to have the same validity.
> 
> If you have any thoughts as to the cause, I can test patches or provide
> output as needed; it would be better if this workaround weren't needed.
> 
> -- 
> Daniel De Graaf
> National Security Agency

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