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

[Jason Wang <jasowang@redhat.com>]

On 2019/9/19 下午7:14, Paolo Bonzini wrote:
> On 19/09/19 09:16, Tian, Kevin wrote:
>>>> 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.
> In general, userspace cannot assume that it's okay to sync just through
> GPA1.  It must sync the host page if *either* GPA1 or GPA2 are marked dirty.


Maybe we need document this somewhere.


>
> The situation really only arises in special cases.  For example,
> 0xfffe0000..0xffffffff and 0xe0000..0xfffff might be the same memory.
>  From "info mtree" before the guest boots:
>
>      0000000000000000-ffffffffffffffff (prio -1, i/o): pci
>        00000000000e0000-00000000000fffff (prio 1, i/o): alias isa-bios
> @pc.bios 0000000000020000-000000000003ffff
>        00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
>
> However, non-x86 machines may have other cases of aliased memory so it's
> a case that you should cover.
>
> Paolo


Any other issue that still need to be covered consider userspace need to 
sync both GPAs?

Thanks