On Thu, Sep 19, 2019 at 05:35:05PM +0800, Jason Wang wrote: > > On 2019/9/19 下午2:32, Yan Zhao wrote: > > On Thu, Sep 19, 2019 at 02:29:54PM +0800, Yan Zhao wrote: > >> On Thu, Sep 19, 2019 at 02:32:03PM +0800, Jason Wang wrote: > >>> 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. > >>> > >> In that case, cannot log dirty according to HPA. > > sorry, it should be "cannot log dirty according to HVA". > > > I think we are discussing the choice between GPA and IOVA, not HVA? > Right. so why do we need to care about HVA to GPA mapping? as long as IOVA to GPA is 1:1, then it's fine.
Thanks Yan > Thanks > > > > > >> because kvm cannot tell whether it's an valid case (the two GPAs are > >> equivalent) > >> or an invalid case (the two GPAs are not equivalent, but with the same > >> HVA value). > >> > >> Right? > >> > >> Thanks > >> Yan > >> > >> > >>> Thanks > >>> > >>> > >>>> Thanks > >>>> Yan > >>>> > >>>> > >>>>> Thanks > >>>>> > >>>>> > >>>>>> Thanks > >>>>>> Yan > >>>>>> > >>>>>>>> Is Qemu doing its own same-content memory > >>>>>>>> merging in GPA level, similar to KSM? > >>>>>>> AFAIK, it doesn't. > >>>>>>> > >>>>>>> Thanks > >>>>>>> > >>>>>>> > >>>>>>>> Thanks > >>>>>>>> Kevin
