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

[hrg <hrgstephen@gmail.com>]

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.