Messages in this thread | | | Subject | Re: [PATCH net V2 4/4] vhost: log dirty page correctly | From | Jason Wang <> | Date | Wed, 26 Dec 2018 13:43:26 +0800 |
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On 2018/12/26 上午12:25, Michael S. Tsirkin wrote: > On Tue, Dec 25, 2018 at 05:43:25PM +0800, Jason Wang wrote: >> On 2018/12/25 上午1:41, Michael S. Tsirkin wrote: >>> On Mon, Dec 24, 2018 at 11:43:31AM +0800, Jason Wang wrote: >>>> On 2018/12/14 下午9:20, Michael S. Tsirkin wrote: >>>>> On Fri, Dec 14, 2018 at 10:43:03AM +0800, Jason Wang wrote: >>>>>> On 2018/12/13 下午10:31, 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
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