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

From: Jason Wang
Date: Wed Dec 26 2018 - 00:43:44 EST

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

Is this what you suggest?

Not really. We already have GPA->HVA, so I suggested a flag to pass

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.

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.

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

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:



2) Qemu GIOVA->GPA

3) Qemu GPA->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?

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.