linux-kernel - Re: [RFT][PATCH v1 6/6] vfio: Replace phys_pfn with phys_page for vfio_pin

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Date:   Sun, 19 Jun 2022 23:37:47 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Nicolin Chen <nicolinc@...dia.com>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, farman@...ux.ibm.com,
        mjrosato@...ux.ibm.com, pasic@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com,
        akrowiak@...ux.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, cohuck@...hat.com,
        kevin.tian@...el.com, jchrist@...ux.ibm.com, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFT][PATCH v1 6/6] vfio: Replace phys_pfn with phys_page for
 vfio_pin_pages()

On Sun, Jun 19, 2022 at 10:51:47PM -0700, Christoph Hellwig wrote:
> On Mon, Jun 20, 2022 at 12:00:46AM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 17, 2022 at 01:54:05AM -0700, Christoph Hellwig wrote:
> > > There is a bunch of code an comments in the iommu type1 code that
> > > suggest we can pin memory that is not page backed.  
> > 
> > AFAIK you can.. The whole follow_pte() mechanism allows raw PFNs to be
> > loaded into the type1 maps and the pin API will happily return
> > them. This happens in almost every qemu scenario because PCI MMIO BAR
> > memory ends up routed down this path.
> 
> Indeed, my read wasn't deep enough.  Which means that we can't change
> the ->pin_pages interface to return a struct pages array, as we don't
> have one for those.

Actually.  gvt requires a struct page, and both s390 seem to require
normal non-I/O, non-remapped kernel pointers.  So I think for the
vfio_pin_pages we can assume that we only want page backed memory and
remove the follow_fault_pfn case entirely.   But we'll probably have
to keep it for the vfio_iommu_replay case that is not tied to
emulated IOMMU drivers.