Messages in this thread | | | Date | Fri, 20 Oct 2023 11:28:58 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory |
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On Fri, Oct 20, 2023 at 04:03:57PM +0200, Lorenzo Pieralisi wrote: > > My point is if this becomes a real world concern we have a solid > > answer on how to resolve it - fix the VFIO driver to have a stronger > > barrier before reset. > > Just to make sure I am parsing this correctly: this case above is > related to a non-PCI VFIO device passthrough where a guest would want to > map the device MMIO at stage-1 with normal-NC memory type (well, let's > say with a memory attribute != device-nGnRE - that combined with the new > stage-2 default might cause transactions ordering/grouping trouble with > eg device resets), correct ?
This is what I have understood was Will's concern, yes.
> IIRC, all requests related to honouring "write-combine" style > stage-1 mappings were for PCI(e) devices but that's as far as what > *I* was made aware of goes.
Yes, this is what I am aware of as well.
Though I do not object to the idea from the VFIO side that platform devices would also have to support NormalNC too.
The theoretical missing peice is that someone would say they have a SOC issue XYZ and thus their VFIO platform devices must fully block NormalNC. I suggest if someone comes with this and really, really wants VFIO, then we could use a VMA flag to indicate that KVM must not upgrade it. Currently I have no knowledge of such a thing existing.
With PCI we've made the argument that if NormalNC is broken unsafe for KVM in the SOC then probably so is Device-*. I think the same basic argument holds for platform devices too. Thus I'm skeptical that someone can come and say they have SOC issue XYZ and NormalNC is broken but Device-* is perfectly safe.
> We will do that, I will start adding the recent discussions to the > new documentation file. Side note: for those who attend LPC it would be > useful to review the resulting documentation together there, it should > happen around v6.7-rc1.
I will be there, let me know
Thanks, Jason
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