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

[Yan Zhao <yan.y.zhao@intel.com>]

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:jasowang@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
> > >>>>
>