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

[Jason Wang]

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