[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [Qemu-devel] [RFC/BUG] xen-mapcache: buggy invalidate map cache?
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). _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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