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Date:   Mon, 14 Sep 2020 10:47:38 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
CC:     Jason Wang <jasowang@redhat.com>, Liu Yi L <yi.l.liu@intel.com>,
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On Mon, Sep 14, 2020 at 03:31:13PM +0200, Jean-Philippe Brucker wrote:

> > Jason suggest something like /dev/sva. There will be a lot of other
> > subsystems that could benefit from this (e.g vDPA).
> 
> Do you have a more precise idea of the interface /dev/sva would provide,
> how it would interact with VFIO and others?  vDPA could transport the
> generic iommu.h structures via its own uAPI, and call the IOMMU API
> directly without going through an intermediate /dev/sva handle.

Prior to PASID IOMMU really only makes sense as part of vfio-pci
because the iommu can only key on the BDF. That can't work unless the
whole PCI function can be assigned. It is hard to see how a shared PCI
device can work with IOMMU like this, so may as well use vfio.

SVA and various vIOMMU models change this, a shared PCI driver can
absoultely work with a PASID that is assigned to a VM safely, and
actually don't need to know if their PASID maps a mm_struct or
something else.

So, some /dev/sva is another way to allocate a PASID that is not 1:1
with mm_struct, as the existing SVA stuff enforces. ie it is a way to
program the DMA address map of the PASID.

This new PASID allocator would match the guest memory layout and
support the IOMMU nesting stuff needed for vPASID.

This is the common code for the complex cases of virtualization with
PASID, shared by all user DMA drivers, including VFIO.

It doesn't make a lot of sense to build a uAPI exclusive to VFIO just
for PASID and vPASID. We already know everything doing user DMA will
eventually need this stuff.

Jason

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From: Jason Gunthorpe <jgg@nvidia.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>
Subject: Re: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing
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On Mon, Sep 14, 2020 at 03:31:13PM +0200, Jean-Philippe Brucker wrote:

> > Jason suggest something like /dev/sva. There will be a lot of other
> > subsystems that could benefit from this (e.g vDPA).
> 
> Do you have a more precise idea of the interface /dev/sva would provide,
> how it would interact with VFIO and others?  vDPA could transport the
> generic iommu.h structures via its own uAPI, and call the IOMMU API
> directly without going through an intermediate /dev/sva handle.

Prior to PASID IOMMU really only makes sense as part of vfio-pci
because the iommu can only key on the BDF. That can't work unless the
whole PCI function can be assigned. It is hard to see how a shared PCI
device can work with IOMMU like this, so may as well use vfio.

SVA and various vIOMMU models change this, a shared PCI driver can
absoultely work with a PASID that is assigned to a VM safely, and
actually don't need to know if their PASID maps a mm_struct or
something else.

So, some /dev/sva is another way to allocate a PASID that is not 1:1
with mm_struct, as the existing SVA stuff enforces. ie it is a way to
program the DMA address map of the PASID.

This new PASID allocator would match the guest memory layout and
support the IOMMU nesting stuff needed for vPASID.

This is the common code for the complex cases of virtualization with
PASID, shared by all user DMA drivers, including VFIO.

It doesn't make a lot of sense to build a uAPI exclusive to VFIO just
for PASID and vPASID. We already know everything doing user DMA will
eventually need this stuff.

Jason
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