linux-kernel - Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

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Date:   Wed, 8 Nov 2023 13:39:24 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

On Wed, Nov 08, 2023 at 08:53:00AM +0000, Tian, Kevin wrote:

> > There are many events related to object in guest memory or controlled
> > by the guest, eg C_BAD_CD and C_BAD_STE. These should be relayed or
> > the emulation is not working well.
> 
> so that's the category of unrecoverable faults?

I haven't looked exhaustively but I do have the impression that the
only recoverable fault is the 'page not present' one.

> btw I can understand C_BAD_CD given it's walked by the physical SMMU
> in nested configuration. But presumably STE is created by the smmu
> driver itself then why would there be an error to be relayed for
> guest STE?

If the guest programs a bad STE it should still generate a C_BAD_STE
even if the mediation SW could theoretically sanitize it (but sanitize
it to what? BLOCKED?). Since we have to forward things like C_BAD_CD
and others we may as well just drop an invalid STE and forward the
event like real HW.

> > > but I didn't get the last piece. If those domains are created by kernel
> > > drivers why would they require a uAPI for userspace to specify fault
> > > capable?
> > 
> > Not to userspace, but a kapi to request a fault capable domain and to
> > supply the fault handler. Eg:
> > 
> >  iommu_domain_alloc_faultable(dev, handler);
> 
> Does it affect SVA too?

Inside the driver the SVA should be constructed out of the same fault
handling infrastructure, but a SVA domain allocation should have a
different allocation function.

Jason