linux-kernel - Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d stage-1 cache invalidation

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Message-ID: <ZXu5whlIGfiq16wF@Asurada-Nvidia>
Date: Thu, 14 Dec 2023 18:28:18 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: "Liu, Yi L" <yi.l.liu@...el.com>, "joro@...tes.org" <joro@...tes.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>, "jgg@...dia.com"
	<jgg@...dia.com>, "robin.murphy@....com" <robin.murphy@....com>,
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	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>, "Duan,
 Zhenzhong" <zhenzhong.duan@...el.com>, "joao.m.martins@...cle.com"
	<joao.m.martins@...cle.com>, "Zeng, Xin" <xin.zeng@...el.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
Subject: Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d
 stage-1 cache invalidation

On Fri, Dec 15, 2023 at 01:50:07AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@...el.com>
> > Sent: Thursday, December 14, 2023 7:27 PM
> >
> > On 2023/11/17 21:18, Yi Liu wrote:> This adds the data structure for
> > flushing iotlb for the nested domain
> >
> > +struct iommu_hwpt_vtd_s1_invalidate {
> > +     __aligned_u64 addr;
> > +     __aligned_u64 npages;
> > +     __u32 flags;
> > +     __u32 __reserved;
> > +     __u32 error;
> > +     __u32 dev_id;
> > +};
> >
> > dev_id is used to report the failed device, userspace should be able to map
> > it to a vRID, and inject it to VM as part of ITE/ICE error.
> >
> > However, I got a problem when trying to get dev_id in cache invalidation
> > path, since this is filled in intel iommu driver. Seems like there is no
> > good way for it. I've below alternatives to move forward, wish you have
> > a look.

> >
> > - Reuse Nicolin's vRID->pRID mapping. If thevRID->pRID mapping is
> > maintained, then intel iommu can report a vRID back to user. But intel
> > iommu driver does not have viommu context, no place to hold the vRID-
> > >pRID
> > mapping. TBH. It may require other reasons to introduce it other than the
> > error reporting need. Anyhow, this requires more thinking and also has
> > dependency even if it is doable in intel side.
> 
> this sounds like a cleaner way to inject knowledge which iommu driver
> requires to find out the user tag. but yes it's a bit weird to introduce
> viommu awareness in intel iommu driver when there is no such thing
> in real hardware.

I think a viommu is defined more like a software object representing
the virtual IOMMU in a VM. Since VT-d has a vIOMMU in a nesting case,
there could be an object for it too?

> and for this error reporting case what we actually require is the
> reverse map i.e. pRID->vRID. Not sure whether we can leverage the
> same RID mapping uAPI as for ARM/AMD but ignore viommu_id
> and then store vRID under device_domain_info. a bit tricky on
> life cycle management and also incompatible with SIOV...

One thing that I am not very clear here: since both vRID and dev_id
are given by the VMM, shouldn't it already know the mapping if the
point is to translate (pRID->)dev_id->vRID?

Thanks
Nicolin