[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 2017/4/13 7:51, Stefano Stabellini wrote: 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@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.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. Yes, I will look into this. 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) _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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