Re: [PATCH net V2 4/4] vhost: log dirty page correctly

[Jason Wang]

On 2018/12/26 äå12:25, Michael S. Tsirkin wrote:
> On Tue, Dec 25, 2018 at 05:43:25PM +0800, Jason Wang wrote:
> > On 2018/12/25 äå1:41, Michael S. Tsirkin wrote:
> > > On Mon, Dec 24, 2018 at 11:43:31AM +0800, Jason Wang wrote:
> > > > On 2018/12/14 äå9:20, Michael S. Tsirkin wrote:
> > > > > On Fri, Dec 14, 2018 at 10:43:03AM +0800, Jason Wang wrote:
> > > > > > On 2018/12/13 äå10:31, Michael S. Tsirkin wrote:
> > > > > > > > Just to make sure I understand this. It looks to me we should:
> > > > > > > >
> > > > > > > > - allow passing GIOVA->GPA through UAPI
> > > > > > > >
> > > > > > > > - cache GIOVA->GPA somewhere but still use GIOVA->HVA in device IOTLB for
> > > > > > > > performance
> > > > > > > >
> > > > > > > > Is this what you suggest?
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > Not really. We already have GPA->HVA, so I suggested a flag to pass
> > > > > > > GIOVA->GPA in the IOTLB.
> > > > > > >
> > > > > > > This has advantages for security since a single table needs
> > > > > > > then to be validated to ensure guest does not corrupt
> > > > > > > QEMU memory.
> > > > > > I wonder how much we can gain through this. Currently, qemu IOMMU gives
> > > > > > GIOVA->GPA mapping, and qemu vhost code will translate GPA to HVA then pass
> > > > > > GIOVA->HVA to vhost. It looks no difference to me.
> > > > > >
> > > > > > Thanks
> > > > > The difference is in security not in performance.  Getting a bad HVA
> > > > > corrupts QEMU memory and it might be guest controlled. Very risky.
> > > > How can this be controlled by guest? HVA was generated from qemu ram blocks
> > > > which is totally under the control of qemu memory core instead of guest.
> > > >
> > > >
> > > > Thanks
> > > It is ultimately under guest influence as guest supplies IOVA->GPA
> > > translations.  qemu translates GPA->HVA and gives the translated result
> > > to the kernel.  If it's not buggy and kernel isn't buggy it's all
> > > fine.
> >
> > If qemu provides buggy GPA->HVA, we can't workaround this. And I don't get
> > the point why we even want to try this. Buggy qemu code can crash itself in
> > many ways.
> >
> >
> > > But that's the approach that was proven not to work in the 20th century.
> > > In the 21st century we are trying defence in depth approach.
> > >
> > > My point is that a single code path that is responsible for
> > > the HVA translations is better than two.
> > So the difference whether or not use memory table information:
> >
> > Current:
> >
> > 1) SET_MEM_TABLE: GPA->HVA
> >
> > 2) Qemu GIOVA->GPA
> >
> > 3) Qemu GPA->HVA
> >
> > 4) IOTLB_UPDATE: GIOVA->HVA
> >
> > If I understand correctly you want to drop step 3 consider it might be buggy
> > which is just 19 lines of code in qemu (vhost_memory_region_lookup()). This
> > will ends up:
> >
> > 1) Do GPA->HVA translation in IOTLB_UPDATE path (I believe we won't want to
> > do it during device IOTLB lookup).
> >
> > 2) Extra bits to enable this capability.
> >
> > So this looks need more codes in kernel than what qemu did in userspace. Is
> > this really worthwhile?
> >
> > Thanks
> So there are several points I would like to make
>
> 1. At the moment without an iommu it is possible to
>     change GPA-HVA mappings and everything keeps working
>     because a change in memory tables flushes the rings.


Interesting, I don't know this before. But when can this happen?


>     However I don't see the iotlb cache being invalidated
>     on that path - did I miss it? If it is not there it's
>     a related minor bug.


It might have a bug. But a question is consider the case without IOMMU. 
We only update mem table (SET_MEM_TABLE), but not vring address. This 
looks like a bug as well?


> 2. qemu already has a GPA. Discarding it and re-calculating
>     when logging is on just seems wrong.
>     However if you would like to *also* keep the HVA in the iotlb
>     to avoid doing extra translations, that sounds like a
>     reasonable optimization.


Yes, traverse GPA->HVA mapping seems unnecessary.


> 3. it also means that the hva->gpa translation only runs
>     when logging is enabled. That is a rarely excercised
>     path so any bugs there will not be caught.


I wonder maybe some kind of unit-test may help here.


> So I really would like us long term to move away from
> hva->gpa translations, keep them for legacy userspace only
> but I don't really mind how we do it.
>
> How about
> - a new flag to pass an iotlb with *both* a gpa and hva
> - for legacy userspace, calculate the gpa on iotlb update
>    so the device then uses a shared code path
>
> what do you think?

I don't object this idea so I can try, just want to figure out why it 
was a must.

Thanks