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

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 12 Apr 2017, Herongguang (Stephen) wrote:
> On 2017/4/12 6:32, Stefano Stabellini wrote:
> > On Tue, 11 Apr 2017, hrg wrote:
> > > On Tue, Apr 11, 2017 at 3:50 AM, Stefano Stabellini
> > > <sstabellini@kernel.org> 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@gmail.com> wrote:
> > > > > > > On Sun, Apr 9, 2017 at 11:52 PM, hrg <hrgstephen@gmail.com> 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.
> 
> Yes, I think so, maybe this is the proper way to fix this.

Would you be up for sending a proper patch and testing it? We cannot call
xen_invalidate_map_cache_entry directly from pci.c though, it would need
to be one of the other functions like address_space_unmap for example.


> > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > index e6b08e1..04f98b7 100644
> > --- a/hw/pci/pci.c
> > +++ b/hw/pci/pci.c
> > @@ -2242,6 +2242,7 @@ static void pci_add_option_rom(PCIDevice *pdev, bool
> > is_default_rom,
> >       }
> >         pci_register_bar(pdev, PCI_ROM_SLOT, 0, &pdev->rom);
> > +    xen_invalidate_map_cache_entry(ptr);
> >   }
> >     static void pci_del_option_rom(PCIDevice *pdev)