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

[Jason Wang <jasowang@redhat.com>]

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.

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
>>
>>