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

[Paolo Bonzini]

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.

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