Re: [Qemu-devel] vhost, iova, and dirty page tracking

[Yan Zhao]

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:address@hidden]
> >>>>>>>>> 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