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From: | Jason Wang |
Subject: | Re: [Qemu-devel] vhost, iova, and dirty page tracking |
Date: | Wed, 18 Sep 2019 14:10:05 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
On 2019/9/18 上午9:44, Tian, Kevin wrote:
From: Jason Wang [mailto:address@hidden] Sent: Tuesday, September 17, 2019 6:36 PM On 2019/9/17 下午4:48, Tian, Kevin wrote:From: Jason Wang [mailto:address@hidden] Sent: Monday, September 16, 2019 4:33 PM On 2019/9/16 上午9:51, Tian, Kevin wrote:Hi, Jason We had a discussion about dirty page tracking in VFIO, when vIOMMU is enabled: https://lists.nongnu.org/archive/html/qemu-devel/2019-09/msg02690.htmlIt's actually a similar model as vhost - Qemu cannot interpose the fast-pathDMAs thus relies on the kernel part to track and report dirty pageinformation.Currently Qemu tracks dirty pages in GFN level, thus demanding atranslationfrom IOVA to GPA. Then the open in our discussion is where thistranslationshould happen. Doing the translation in kernel implies a device iotlbflavor,which is what vhost implements today. It requires potentially largetrackingstructures in the host kernel, but leveraging the existing log_sync flowinQemu.On the other hand, Qemu may perform log_sync for every removal ofIOVAmapping and then do the translation itself, then avoiding the GPAawarenessin the kernel side. It needs some change to current Qemu log-sync flow,andmay bring more overhead if IOVA is frequently unmapped. So we'd like to hear about your opinions, especially about how youcamedown to the current iotlb approach for vhost.We don't consider too much in the point when introducing vhost. And before IOTLB, vhost has already know GPA through its mem table (GPA->HVA). So it's nature and easier to track dirty pages at GPA level then it won't any changes in the existing ABI.This is the same situation as VFIO.For VFIO case, the only advantages of using GPA is that the log can then be shared among all the devices that belongs to the VM. Otherwise syncing through IOVA is cleaner.I still worry about the potential performance impact with this approach. In current mdev live migration series, there are multiple system calls involved when retrieving the dirty bitmap information for a given memory range.I haven't took a deep look at that series. Technically dirty bitmap could be shared between device and driver, then there's no system call in synchronization.That series require Qemu to tell the kernel about the information about queried region (start, number, and page_size), read the information about the dirty bitmap (offset, size) and then read the dirty bitmap.
Any pointer to that series, I can only find a "mdev live migration support with vfio-mdev-pci" from Liu Yi without actual codes.
Although the bitmap can be mmaped thus shared, earlier reads/writes are conducted by pread/pwrite system calls. This design is fine for current log_dirty implementation, where dirty bitmap is synced in every pre-copy round. But to do it for every IOVA unmap, it's definitely over-killed.IOVA mappings might be changed frequently. Though one may argue that frequent IOVA change already has bad performance, it's still not good to introduce further non-negligible overhead in such situation.Yes, it depends on the behavior of vIOMMU driver, e.g the frequency and granularity of the flushing.On the other hand, I realized that adding IOVA awareness in VFIO is actually easy. Today VFIO already maintains a full list of IOVA and its associated HVA in vfio_dma structure, according to VFIO_MAP and VFIO_UNMAP. As long as we allow the latter two operations to accept another parameter (GPA), IOVA->GPA mapping can be naturally cached in existing vfio_dma objects.Note that the HVA to GPA mapping is not an 1:1 mapping. One HVA range could be mapped to several GPA ranges.This is fine. Currently vfio_dma maintains IOVA->HVA mapping. btw under what condition HVA->GPA is not 1:1 mapping? I didn't realize it.
I don't remember the details e.g memory region alias? And neither kvm nor kvm API does forbid this if my memory is correct.
Those objects are always updated according to MAP and UNMAP ioctls to be up-to-date. Qemu then uniformly retrieves the VFIO dirty bitmap for the entire GPA range in every pre-copy round, regardless of whether vIOMMU is enabled. There is no need of another IOTLB implementation, with the main ask on a v2 MAP/UNMAP interface.Or provide GPA to HVA mapping as vhost did. But a question is, I believe device can only do dirty page logging through IOVA. So how do you handle the case when IOVA is removed in this case?That's why a log_sync is required each time when IOVA is unmapped, in Alex's thought. Thanks Kevin
Ok. Thanks
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