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

[Jason Wang <jasowang@redhat.com>]

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:jasowang@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: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.
>>>>
>>> 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