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".
> 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 > > >>>> >
