On Fri, Sep 20, 2019 at 09:15:40AM +0800, Jason Wang wrote: > > On 2019/9/19 下午10:06, Michael S. Tsirkin wrote: > > On Thu, Sep 19, 2019 at 05:37:48PM +0800, Jason Wang wrote: > > > On 2019/9/19 下午3:16, Tian, Kevin wrote: > > > > +Paolo to help clarify here. > > > > > > > > > From: Jason Wang [mailto:jasow...@redhat.com] > > > > > Sent: Thursday, September 19, 2019 2:32 PM > > > > > > > > > > > > > > > On 2019/9/19 下午2:17, Yan Zhao wrote: > > > > > > On Thu, Sep 19, 2019 at 02:09:53PM +0800, Jason Wang wrote: > > > > > > > On 2019/9/19 下午1:28, Yan Zhao wrote: > > > > > > > > On Thu, Sep 19, 2019 at 09:05:12AM +0800, Jason Wang wrote: > > > > > > > > > On 2019/9/18 下午4:37, Tian, Kevin wrote: > > > > > > > > > > > From: Jason Wang [mailto:jasow...@redhat.com] > > > > > > > > > > > Sent: Wednesday, September 18, 2019 2:10 PM > > > > > > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > > I checked https://qemu.weilnetz.de/doc/devel/memory.html, > > > > > > > > > > which > > > > > > > > > > provides an example of aliased layout. However, its > > > > > > > > > > aliasing is all > > > > > > > > > > 1:1, instead of N:1. From guest p.o.v every writable GPA > > > > > > > > > > implies an > > > > > > > > > > unique location. Why would we hit the situation where > > > > > > > > > > multiple > > > > > > > > > > write-able GPAs are mapped to the same HVA (i.e. same > > > > > > > > > > physical > > > > > > > > > > memory location)? > > > > > > > > > I don't know, just want to say current API does not forbid > > > > > > > > > this. So we > > > > > > > > > probably need to take care it. > > > > > > > > > > > > > > > > > yes, in KVM API level, it does not forbid two slots to have the > > > > > > > > same > > > > > HVA(slot->userspace_addr). > > > > > > > > But > > > > > > > > (1) there's only one kvm instance for each vm for each qemu > > > > > > > > process. > > > > > > > > (2) all ramblock->host (corresponds to HVA and > > > > > > > > slot->userspace_addr) > > > > > in one qemu > > > > > > > > process is non-overlapping as it's obtained from mmmap(). > > > > > > > > (3) qemu ensures two kvm slots will not point to the same > > > > > > > > section of > > > > > one ramblock. > > > > > > > > So, as long as kvm instance is not shared in two processes, and > > > > > > > > there's no bug in qemu, we can assure that HVA to GPA is 1:1. > > > > > > > Well, you leave this API for userspace, so you can't assume qemu > > > > > > > is the > > > > > > > only user or any its behavior. If you had you should limit it in > > > > > > > the API > > > > > > > level instead of open window for them. > > > > > > > > > > > > > > > > > > > > > > But even if there are two processes operating on the same kvm > > > > > instance > > > > > > > > and manipulating on memory slots, adding an extra GPA along side > > > > > current > > > > > > > > IOVA & HVA to ioctl VFIO_IOMMU_MAP_DMA can still let driver > > > > > > > > knows > > > > > the > > > > > > > > right IOVA->GPA mapping, right? > > > > > > > It looks fragile. Consider HVA was mapped to both GPA1 and GPA2. > > > > > Guest > > > > > > > maps IOVA to GPA2, so we have IOVA GPA2 HVA in the new ioctl and > > > > > then > > > > > > > log through GPA2. If userspace is trying to sync through GPA1, it > > > > > > > will > > > > > > > miss the dirty page. So for safety we need log both GPA1 and > > > > > > > GPA2. (See > > > > > > > what has been done in log_write_hva() in vhost.c). The only way > > > > > > > to do > > > > > > > that is to maintain an independent HVA to GPA mapping like what > > > > > > > KVM > > > > > or > > > > > > > vhost did. > > > > > > > > > > > > > why GPA1 and GPA2 should be both dirty? > > > > > > even they have the same HVA due to overlaping virtual address space > > > > > > in > > > > > > two processes, they still correspond to two physical pages. > > > > > > don't get what's your meaning :) > > > > > The point is not leave any corner case that is hard to debug or fix in > > > > > the future. > > > > > > > > > > Let's just start by a single process, the API allows userspace to maps > > > > > HVA to both GPA1 and GPA2. Since it knows GPA1 and GPA2 are > > > > > equivalent, > > > > > it's ok to sync just through GPA1. That means if you only log GPA2, it > > > > > won't work. > > > > > > > > > I noted KVM itself doesn't consider such situation (one HVA is mapped > > > > to multiple GPAs), when doing its dirty page tracking. If you look at > > > > kvm_vcpu_mark_page_dirty, it simply finds the unique memslot which > > > > contains the dirty gfn and then set the dirty bit within that slot. It > > > > doesn't attempt to walk all memslots to find out any other GPA which > > > > may be mapped to the same HVA. > > > > > > > > So there must be some disconnect here. let's hear from Paolo first and > > > > understand the rationale behind such situation. > > > > > > Neither did vhost when IOTLB is disabled. And cc Michael who points out > > > this > > > issue at the beginning. > > > > > > Thanks > > > > > > > > > > Thanks > > > > Kevin > > Yes, we fixed with a kind of a work around, at the time I proposed > > a new interace to fix it fully. I don't think we ever got around > > to implementing it - right? > > > Paolo said userspace just need to sync through all GPAs, so my understanding > is that work around is ok by redundant, so did the API you proposed. > Anything I miss? > > Thanks
I just feel an extra lookup is awkward. We don't benchmark the speed during migration right now but it's something we might care about down the road. HTH -- MST
