From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56776) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cyDRF-0002T1-KN for qemu-devel@nongnu.org; Wed, 12 Apr 2017 04:15:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cyDRB-0001px-Hl for qemu-devel@nongnu.org; Wed, 12 Apr 2017 04:15:41 -0400 Received: from szxga03-in.huawei.com ([45.249.212.189]:3363 helo=dggrg03-dlp.huawei.com) by eggs.gnu.org with esmtps (TLS1.0:RSA_ARCFOUR_SHA1:16) (Exim 4.71) (envelope-from ) id 1cyDRA-0001mK-Mp for qemu-devel@nongnu.org; Wed, 12 Apr 2017 04:15:37 -0400 References: From: "Herongguang (Stephen)" Message-ID: <58EDE1E4.2090003@huawei.com> Date: Wed, 12 Apr 2017 16:14:28 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [RFC/BUG] xen-mapcache: buggy invalidate map cache? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefano Stabellini , hrg Cc: Anthony PERARD , xen-devel@lists.xensource.com, qemu-devel@nongnu.org, Jun Nakajima , Alexander Graf , xen-devel@lists.xenproject.org, xen-devel@lists.xen.org, wangxinxin.wang@huawei.com 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 >> 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 wrote: >>>>>> On Sun, Apr 9, 2017 at 11:52 PM, hrg 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. > > > 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)