linux-kernel - Re: [PATCH 2/4] vhost-vdpa: reset vendor specific mapping to initial state in .release

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Date:   Thu, 19 Oct 2023 15:57:51 -0700
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Eugenio Perez Martin <eperezma@...hat.com>, mst@...hat.com,
        xuanzhuo@...ux.alibaba.com, dtatulea@...dia.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] vhost-vdpa: reset vendor specific mapping to initial
 state in .release



On 10/17/2023 10:27 PM, Jason Wang wrote:
>>>    If we do
>>> this without a negotiation, IOTLB will not be clear but the Qemu will
>>> try to re-program the IOTLB after reset. Which will break?
>>>
>>> 1) stick the exact old behaviour with just one line of check
>> It's not just one line of check here, the old behavior emulation has to
>> be done as Eugenio illustrated in the other email.
> For vhost-vDPA it's just
>
> if (IOTLB_PERSIST is acked by userspace)
>      reset_map()
... and this reset_map in vhost_vdpa_cleanup can't be negotiable 
depending on IOTLB_PERSIST. Consider the case where user switches to 
virtio-vdpa after an older userspace using vhost-vdpa finished running. 
Even with buggy_virtio_reset_map in place it's unwarranted the vendor 
IOMMU can get back to the default state, e.g. ending with 1:1 
passthrough mapping. If not doing this unconditionally it will get a big 
chance to break userspace.

-Siwei

>
> For parent, it's somehow similar:
>
> during .reset()
>
> if (IOTLB_PERSIST is not acked by userspace)
>          reset_vendor_mappings()
>
> Anything I missed here?