On Tue, Dec 25, 2018 at 05:43:25PM +0800, Jason Wang wrote:
On 2018/12/25 äå1:41, Michael S. Tsirkin wrote:So there are several points I would like to make
On Mon, Dec 24, 2018 at 11:43:31AM +0800, Jason Wang wrote:
On 2018/12/14 äå9:20, Michael S. Tsirkin wrote:It is ultimately under guest influence as guest supplies IOVA->GPA
On Fri, Dec 14, 2018 at 10:43:03AM +0800, Jason Wang wrote:How can this be controlled by guest? HVA was generated from qemu ram blocks
On 2018/12/13 äå10:31, Michael S. Tsirkin wrote:The difference is in security not in performance. Getting a bad HVA
I wonder how much we can gain through this. Currently, qemu IOMMU givesJust to make sure I understand this. It looks to me we should:Not really. We already have GPA->HVA, so I suggested a flag to pass
- allow passing GIOVA->GPA through UAPI
- cache GIOVA->GPA somewhere but still use GIOVA->HVA in device IOTLB for
Is this what you suggest?
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
GIOVA->GPA mapping, and qemu vhost code will translate GPA to HVA then pass
GIOVA->HVA to vhost. It looks no difference to me.
corrupts QEMU memory and it might be guest controlled. Very risky.
which is totally under the control of qemu memory core instead of guest.
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
But that's the approach that was proven not to work in the 20th century.So the difference whether or not use memory table information:
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.
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.Â Is
this really worthwhile?
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.
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.
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
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.
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.
- 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?