linux-kernel - Re: [PATCH] vhost/vsock: add IOTLB API support

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Date:   Fri, 30 Oct 2020 19:44:43 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     mst@...hat.com, netdev@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost/vsock: add IOTLB API support


On 2020/10/30 下午6:54, Stefano Garzarella wrote:
> On Fri, Oct 30, 2020 at 06:02:18PM +0800, Jason Wang wrote:
>>
>> On 2020/10/30 上午1:43, Stefano Garzarella wrote:
>>> This patch enables the IOTLB API support for vhost-vsock devices,
>>> allowing the userspace to emulate an IOMMU for the guest.
>>>
>>> These changes were made following vhost-net, in details this patch:
>>> - exposes VIRTIO_F_ACCESS_PLATFORM feature and inits the iotlb
>>>   device if the feature is acked
>>> - implements VHOST_GET_BACKEND_FEATURES and
>>>   VHOST_SET_BACKEND_FEATURES ioctls
>>> - calls vq_meta_prefetch() before vq processing to prefetch vq
>>>   metadata address in IOTLB
>>> - provides .read_iter, .write_iter, and .poll callbacks for the
>>>   chardev; they are used by the userspace to exchange IOTLB messages
>>>
>>> This patch was tested with QEMU and a patch applied [1] to fix a
>>> simple issue:
>>>     $ qemu -M q35,accel=kvm,kernel-irqchip=split \
>>>            -drive file=fedora.qcow2,format=qcow2,if=virtio \
>>>            -device intel-iommu,intremap=on \
>>>            -device vhost-vsock-pci,guest-cid=3,iommu_platform=on
>>
>>
>> Patch looks good, but a question:
>>
>> It looks to me you don't enable ATS which means vhost won't get any 
>> invalidation request or did I miss anything?
>>
>
> You're right, I didn't see invalidation requests, only miss and updates.
> Now I have tried to enable 'ats' and 'device-iotlb' but I still don't 
> see any invalidation.
>
> How can I test it? (Sorry but I don't have much experience yet with 
> vIOMMU)


I guess it's because the batched unmap. Maybe you can try to use 
"intel_iommu=strict" in guest kernel command line to see if it works.

Btw, make sure the qemu contains the patch [1]. Otherwise ATS won't be 
enabled for recent Linux Kernel in the guest.

Thanks

[1] https://patchew.org/QEMU/20200909081731.24688-1-jasowang@redhat.com/

>
> Thanks,
> Stefano
>