Re: [Qemu-devel] intel-iommu and vhost: Do we need 'device-iotlb' and 'ats'?

[Jintack Lim]

Hi Eric,

On Mon, Feb 26, 2018 at 5:14 AM, Auger Eric <address@hidden> wrote:
> Hi Jintack,
>
> On 21/02/18 05:03, Jintack Lim wrote:
>> Hi,
>>
>> I'm using vhost with the virtual intel-iommu, and this page[1] shows
>> the QEMU command line example.
>>
>> qemu-system-x86_64 -M q35,accel=kvm,kernel-irqchip=split -m 2G \
>>                    -device intel-iommu,intremap=on,device-iotlb=on \
>>                    -device ioh3420,id=pcie.1,chassis=1 \
>>                    -device
>> virtio-net-pci,bus=pcie.1,netdev=net0,disable-legacy=on,disable-modern=off,iommu_platform=on,ats=on
>> \
>>                    -netdev tap,id=net0,vhostforce \
>>                    $IMAGE_PATH
>>
>> I wonder what's the impact of using device-iotlb and ats options as
>> they are described necessary.
>>
>> In my understanding, vhost in the kernel only looks at
>> VIRTIO_F_IOMMU_PLATFORM, and when it is set, vhost uses a
>> device-iotlb. In addition, vhost and QEMU communicate using vhost_msg
>> basically to cache mappings correctly in the vhost, so I wonder what's
>> the role of ats in this case.
>>
>> A related question is that if we use SMMU emulation[2] on ARM without
>> those options, does vhost cache mappings as if it has a device-iotlb?
>> (I guess this is the case.)
> vsmmuv3 emulation code does not support ATS at the moment. vhost support
> is something different. As Peter explained it comes with the capability
> of the virtio device to register unmap notifiers. Those notifiers get
> called each time there are TLB invalidation commands. That way the
> in-kernel vhost cache can be invalidated. vhost support was there until
> vsmmuv3 v7. With latest versions, I removed it to help reviewers
> concentrate on the root functionality. However I will send it to you
> based on v9.

Thanks, Eric. I'm happy to take a look at those patches!

Thanks,
Jintack

>
> Thanks
>
> Eric
>>
>> I'm pretty new to QEMU code, so I might be missing something. Can
>> somebody shed some light on it?
>>
>> [1] https://wiki.qemu.org/Features/VT-d
>> [2] http://lists.nongnu.org/archive/html/qemu-devel/2018-02/msg04736.html
>>
>> Thanks,
>> Jintack
>>
>>
>