Messages in this thread | | | Date | Fri, 17 Apr 2020 08:50:54 +0200 | From | Christoph Hellwig <> | Subject | Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain |
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On Thu, Apr 16, 2020 at 03:40:38PM +0800, Lu Baolu wrote: >> description. I'd need to look at the final code, but it seems like >> this will still cause bounce buffering instead of using dynamic >> mapping, which still seems like an awful idea. > > Yes. If the user chooses to use identity domain by default through > kernel command, identity domain will be applied for all devices. For > those devices with limited addressing capability, bounce buffering will > be used when they try to access the memory beyond their address > capability. This won't cause any kernel regression as far as I can see. > > Switching domain during runtime with drivers loaded will cause real > problems as I said in the commit message. That's the reason why I am > proposing to remove it. If we want to keep it, we have to make sure that > switching domain for one device should not impact other devices which > share the same domain with it. Furthermore, it's better to implement it > in the generic layer to keep device driver behavior consistent on all > architectures.
I don't disagree with the technical points. What I pointed out is that
a) the actual technical change is not in the commit log, which it should be b) that I still think taking away the ability to dynamically map devices in the identify domain after all the time we allowed for that is going to cause nasty regressions.
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