Re: [Qemu-devel] [PATCH] intel-iommu: optimize nodmar memory regions

[Paolo Bonzini]

On 13/03/19 12:45, Sergio Lopez wrote:
> 
> Peter Xu writes:
> 
>> Previously we have per-device system memory aliases when DMAR is
>> disabled by the system.  It will slow the system down if there are
>> lots of devices especially when DMAR is disabled, because each of the
>> aliased system address space will contain O(N) slots, and rendering
>> such N address spaces will be O(N^2) complexity.
>>
>> This patch introduces a shared nodmar memory region and for each
>> device we only create an alias to the shared memory region.  With the
>> aliasing, QEMU memory core API will be able to detect when devices are
>> sharing the same address space (which is the nodmar address space)
>> when rendering the FlatViews and the total number of FlatViews can be
>> dramatically reduced when there are a lot of devices.
>>
>> Suggested-by: Paolo Bonzini <address@hidden>
>> Signed-off-by: Peter Xu <address@hidden>
>> ---
>>
>> Hi, Sergio,
>>
>> This patch implements the optimization that Paolo proposed in the
>> other thread.  Would you please try this patch to see whether it could
>> help for your case?  Thanks,
> 
> Hi,
> 
> I've just gave a try and it fixes the issue here. The number of
> FlatViews goes down from 119 to 4, and the initialization time for PCI
> devices on the Guest is back to normal levels.

You must be using "iommu=pt" then.  Can you also test performance
without it?  It should be fine even if the number of FlatViews goes back
to 119.

Paolo