linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 09:11:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

在 2021/6/3 下午9:09, Jason Gunthorpe 写道:
> On Thu, Jun 03, 2021 at 10:52:51AM +0800, Jason Wang wrote:
>
>> Basically, we don't want to bother with pseudo KVM device like what VFIO
>> did. So for simplicity, we rules out the IOMMU that can't enforce coherency
>> in vhost-vDPA if the parent purely depends on the platform IOMMU:
> VDPA HW cannot issue no-snoop TLPs in the first place.

Note that virtio/vDPA is not necessarily a PCI device.

>
> virtio does not define a protocol to discover such a functionality,

Actually we had:

VIRTIO_F_ACCESS_PLATFORM(33)
This feature indicates that the device can be used on a platform where 
device access to data in memory is limited and/or translated. E.g. this 
is the case if the device can be located behind an IOMMU that translates 
bus addresses from the device into physical addresses in memory, if the 
device can be limited to only access certain memory addresses or if 
special commands such as a cache flush can be needed to synchronise data 
in memory with the device.

> nor do any virtio drivers implement the required platform specific
> cache flushing to make no-snoop TLPs work.

I don't get why virtio drivers needs to do that. I think DMA API should 
hide those arch/platform specific stuffs from us.

>
> It is fundamentally part of the virtio HW PCI API that a device vendor
> cannot alter.

The spec doesn't forbid this, and it just leave the detection and action 
to the driver in a platform specific way.

Thanks

>
> Basically since we already know that the virtio kernel drivers do not
> call the cache flush instruction we don't need the weird KVM logic to
> turn it on at all.
>
> Enforcing no-snoop at the IOMMU here is redundant/confusing.
>
> Jason
>