Re: [Qemu-devel] [PATCH v4 10/10] vhost: iommu: cache static mapping if there is

[Jason Wang <jasowang@redhat.com>]

On 2017年05月20日 00:55, Michael S. Tsirkin wrote:
> On Fri, May 19, 2017 at 11:19:49AM +0800, Peter Xu wrote:
>> This patch pre-heat vhost iotlb cache when passthrough mode enabled.
>>
>> Sometimes, even if user specified iommu_platform for vhost devices,
>> IOMMU might still be disabled. One case is passthrough mode in VT-d
>> implementation. We can detect this by observing iommu_list. If it's
>> empty, it means IOMMU translation is disabled, then we can actually
>> pre-heat the translation (it'll be static mapping then) by first
>> invalidating all IOTLB, then cache existing memory ranges into vhost
>> backend iotlb using 1:1 mapping.
>>
>> Signed-off-by: Peter Xu<peterx@redhat.com>
> I don't really understand. Is this a performance optimization?
> Can you post some #s please?
>
> Also, if it's PT, can't we bypass iommu altogether?

The problem is, since device could be moved between domains, which means 
we need new notifier to notify vhost to enable or disable IOMMU_PLATFORM.

> That would be
> even faster ...
>

Should be the same (except for the first access in no CM mode), we pass 
and use vhost_memory_regions as what we've used for non iommu case.

Thanks