Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendor zhaoxin

[Paolo Bonzini]

On 19/10/20 21:02, Alex Williamson wrote:
>> For KVM we were thinking of changing the whole
>> memory map with a single ioctl, but that's much easier because KVM
>> builds its page tables lazily. It would be possible for the IOMMU too
>> but it would require a relatively complicated comparison of the old and
>> new memory maps in the kernel.
>
> We can only build IOMMU page tables lazily if we get faults, which we
> generally don't.  We also cannot atomically update IOMMU page tables
> relative to a device,

Yeah, I didn't mean building IOMMU page tables lazily, rather replacing
the whole VFIO memory map with a single ioctl.

I don't think that requires atomic updates of the IOMMU page table root,
but it would require atomic updates of IOMMU page table entires; VFIO
would compare the old and new memory map and modify the page tables when
it sees a difference.  Is that possible?

Paolo

> so "housekeeping" updates of mappings to (I
> assume) consolidate KVM memory slots doesn't work so well when the
> device is still running.  Stopping the device via something like the
> bus-master enable bit also sounds like a whole set of problems itself.
> I assume these simplified mappings also reduce our resolution for later
> unmaps, which isn't necessarily a win for an assigned device either if
> it exposes the race again each boot.
> 
> Maybe the question is why we don't see these errors more regularly, is
> there something unique about the memory layout of this platform versus
> others that causes larger memory regions to be coalesced together only
> to be later unmapped and provide more exposure to this issue?  Thanks,