linux-kernel - Re: [PATCH v4 15/32] vfio: introduce KVM-owned IOMMU type

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Message-ID: <494fe1b2-cf44-23a6-a494-52a44041ad79@linux.ibm.com>
Date:   Thu, 17 Mar 2022 14:51:34 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     borntraeger@...ux.ibm.com, linux-s390@...r.kernel.org,
        alex.williamson@...hat.com, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, joro@...tes.org, will@...nel.org,
        pbonzini@...hat.com, corbet@....net, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 15/32] vfio: introduce KVM-owned IOMMU type

On 3/15/22 1:25 PM, Jason Gunthorpe wrote:
> On Tue, Mar 15, 2022 at 12:29:02PM -0400, Matthew Rosato wrote:
>> On 3/15/22 10:38 AM, Jason Gunthorpe wrote:
>>> On Tue, Mar 15, 2022 at 09:49:01AM -0400, Matthew Rosato wrote:
>>>
>>>> The rationale for splitting steps 1 and 2 are that VFIO_SET_IOMMU doesn't
>>>> have a mechanism for specifying more than the type as an arg, no?  Otherwise
>>>> yes, you could specify a kvm fd at this point and it would have some other
>>>> advantages (e.g. skip notifier).  But we still can't use the IOMMU for
>>>> mapping until step 3.
>>>
>>> Stuff like this is why I'd be much happier if this could join our
>>> iommfd project so we can have clean modeling of the multiple iommu_domains.
>>>
>>
>> I'd certainly be willing to collaborate so feel free to loop me in on the
>> discussions;
> 
> Sure, I have you on my list. I've been waiting for Eric to get a bit
> further along on his ARM work so you have something appropriate to
> look at.
> 
> In the mean time you can certainly work out the driver details as
> you've been doing here and hacking through VFIO. The iommu_domain
> logic is the big work item in this series, not the integration with
> the uAPI.
>

A subset of this series (enabling some s390x firmware-assist facilities) 
is not tied to the iommu and would still provide value while continuing 
to use vfio_iommu_type1 for all mapping -- so I think I'll look into a 
next version that shrinks down to that subset (+ re-visit the setup API).

Separate from that, I will continue looking at implementing the nested 
iommu_domain logic for s390, and continue to hack through VFIO for now. 
I'll use an RFC series when I have something more to look at, likely 
starting with the fully-pinned guest as you suggest; ultimately I'm 
interested in both scenarios (pinned kvm guest & dynamic pinning during 
shadow)

Thanks,
Matt