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

[Jason Wang <jasowang@redhat.com>]

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: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".


I think we are discussing the choice between GPA and IOVA, not HVA?

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