linux-kernel - Re: [RFC v2] /dev/iommu uAPI proposal

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Date:   Thu, 15 Jul 2021 10:48:36 -0700
From:   "Raj, Ashok" <ashok.raj@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Shenming Lu <lushenming@...wei.com>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "wanghaibin.wang@...wei.com" <wanghaibin.wang@...wei.com>
Subject: Re: [RFC v2] /dev/iommu uAPI proposal

On Thu, Jul 15, 2021 at 02:18:26PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 15, 2021 at 09:21:41AM -0700, Raj, Ashok wrote:
> > On Thu, Jul 15, 2021 at 12:23:25PM -0300, Jason Gunthorpe wrote:
> > > On Thu, Jul 15, 2021 at 06:57:57AM -0700, Raj, Ashok wrote:
> > > > On Thu, Jul 15, 2021 at 09:48:13AM -0300, Jason Gunthorpe wrote:
> > > > > On Thu, Jul 15, 2021 at 06:49:54AM +0000, Tian, Kevin wrote:
> > > > > 
> > > > > > No. You are right on this case. I don't think there is a way to 
> > > > > > differentiate one mdev from the other if they come from the
> > > > > > same parent and attached by the same guest process. In this
> > > > > > case the fault could be reported on either mdev (e.g. the first
> > > > > > matching one) to get it fixed in the guest.
> > > > > 
> > > > > If the IOMMU can't distinguish the two mdevs they are not isolated
> > > > > and would have to share a group. Since group sharing is not supported
> > > > > today this seems like a non-issue
> > > > 
> > > > Does this mean we have to prevent 2 mdev's from same pdev being assigned to
> > > > the same guest? 
> > > 
> > > No, it means that the IOMMU layer has to be able to distinguish them.
> > 
> > Ok, the guest has no control over it, as it see 2 separate pci devices and
> > thinks they are all different.
> > 
> > Only time when it can fail is during the bind operation. From guest
> > perspective a bind in vIOMMU just turns into a write to local table and a
> > invalidate will cause the host to update the real copy from the shadow.
> > 
> > There is no way to fail the bind? and Allocation of the PASID is also a
> > separate operation and has no clue how its going to be used in the guest.
> 
> You can't attach the same RID to the same PASID twice. The IOMMU code
> should prevent this.
> 
> As we've talked about several times, it seems to me the vIOMMU
> interface is misdesigned for the requirements you have. The hypervisor
> should have a role in allocating the PASID since there are invisible
> hypervisor restrictions. This is one of them.

Allocating a PASID is a separate step from binding, isn't it? In vt-d we
have a virtual command interface that can fail an allocation of PASID. But
which device its bound to is a dynamic thing that only gets at bind_mm()
right?

> 
> > Do we have any isolation requirements here? its the same process. So if the
> > page-request it sent to guest and even if you report it for mdev1, after
> > the PRQ is resolved by guest, the request from mdev2 from the same guest
> > should simply work?
> 
> I think we already talked about this and said it should not be done.

I get the should not be done, I'm wondering where should that be
implemented?