Re: [Xen-devel] [Qemu-devel] [RFC/BUG] xen-mapcache: buggy invalidate map cache?

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 12 Apr 2017, Alexey G wrote:
> On Tue, 11 Apr 2017 15:32:09 -0700 (PDT)
> Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
> > On Tue, 11 Apr 2017, hrg wrote:
> > > On Tue, Apr 11, 2017 at 3:50 AM, Stefano Stabellini
> > > <sstabellini@xxxxxxxxxx> wrote:  
> > > > On Mon, 10 Apr 2017, Stefano Stabellini wrote:  
> > > >> On Mon, 10 Apr 2017, hrg wrote:  
> > > >> > On Sun, Apr 9, 2017 at 11:55 PM, hrg <hrgstephen@xxxxxxxxx> wrote:  
> > > >> > > On Sun, Apr 9, 2017 at 11:52 PM, hrg <hrgstephen@xxxxxxxxx> wrote: 
> > > >> > >  
> > > >> > >> Hi,
> > > >> > >>
> > > >> > >> In xen_map_cache_unlocked(), map to guest memory maybe in
> > > >> > >> entry->next instead of first level entry (if map to rom other than
> > > >> > >> guest memory comes first), while in xen_invalidate_map_cache(),
> > > >> > >> when VM ballooned out memory, qemu did not invalidate cache 
> > > >> > >> entries
> > > >> > >> in linked list(entry->next), so when VM balloon back in memory,
> > > >> > >> gfns probably mapped to different mfns, thus if guest asks device
> > > >> > >> to DMA to these GPA, qemu may DMA to stale MFNs.
> > > >> > >>
> > > >> > >> So I think in xen_invalidate_map_cache() linked lists should also 
> > > >> > >> be
> > > >> > >> checked and invalidated.
> > > >> > >>
> > > >> > >> What’s your opinion? Is this a bug? Is my analyze correct?  
> > > >>
> > > >> Yes, you are right. We need to go through the list for each element of
> > > >> the array in xen_invalidate_map_cache. Can you come up with a patch?  
> > > >
> > > > I spoke too soon. In the regular case there should be no locked mappings
> > > > when xen_invalidate_map_cache is called (see the DPRINTF warning at the
> > > > beginning of the functions). Without locked mappings, there should never
> > > > be more than one element in each list (see xen_map_cache_unlocked:
> > > > entry->lock == true is a necessary condition to append a new entry to
> > > > the list, otherwise it is just remapped).
> > > >
> > > > Can you confirm that what you are seeing are locked mappings
> > > > when xen_invalidate_map_cache is called? To find out, enable the DPRINTK
> > > > by turning it into a printf or by defininig MAPCACHE_DEBUG.  
> > > 
> > > In fact, I think the DPRINTF above is incorrect too. In
> > > pci_add_option_rom(), rtl8139 rom is locked mapped in
> > > pci_add_option_rom->memory_region_get_ram_ptr (after
> > > memory_region_init_ram). So actually I think we should remove the
> > > DPRINTF warning as it is normal.  
> > 
> > Let me explain why the DPRINTF warning is there: emulated dma operations
> > can involve locked mappings. Once a dma operation completes, the related
> > mapping is unlocked and can be safely destroyed. But if we destroy a
> > locked mapping in xen_invalidate_map_cache, while a dma is still
> > ongoing, QEMU will crash. We cannot handle that case.
> > 
> > However, the scenario you described is different. It has nothing to do
> > with DMA. It looks like pci_add_option_rom calls
> > memory_region_get_ram_ptr to map the rtl8139 rom. The mapping is a
> > locked mapping and it is never unlocked or destroyed.
> > 
> > It looks like "ptr" is not used after pci_add_option_rom returns. Does
> > the append patch fix the problem you are seeing? For the proper fix, I
> > think we probably need some sort of memory_region_unmap wrapper or maybe
> > a call to address_space_unmap.
> 
> Hmm, for some reason my message to the Xen-devel list got rejected but was 
> sent
> to Qemu-devel instead, without any notice. Sorry if I'm missing something
> obvious as a list newbie.
> 
> Stefano, hrg,
> 
> There is an issue with inconsistency between the list of normal 
> MapCacheEntry's
> and their 'reverse' counterparts - MapCacheRev's in locked_entries.
> When bad situation happens, there are multiple (locked) MapCacheEntry
> entries in the bucket's linked list along with a number of MapCacheRev's. And
> when it comes to a reverse lookup, xen-mapcache picks the wrong entry from the
> first list and calculates a wrong pointer from it which may then be caught 
> with
> the "Bad RAM offset" check (or not). Mapcache invalidation might be related to
> this issue as well I think.
> 
> I'll try to provide a test code which can reproduce the issue from the
> guest side using an emulated IDE controller, though it's much simpler to 
> achieve
> this result with an AHCI controller using multiple NCQ I/O commands. So far 
> I've
> seen this issue only with Windows 7 (and above) guest on AHCI, but any block 
> I/O
> DMA should be enough I think.

That would be helpful. Please see if you can reproduce it after fixing
the other issue (http://marc.info/?l=qemu-devel&m=149195042500707&w=2).

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