linux-kernel - Re: [PATCH v1 1/2] vfio:iommu: Use capabilities do report IOMMU informations

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Date:   Wed, 9 Jan 2019 12:43:37 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        walling@...ux.ibm.com, cohuck@...hat.com, david@...hat.com,
        pasic@...ux.ibm.com, thuth@...hat.com,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH v1 1/2] vfio:iommu: Use capabilities do report IOMMU
 informations

On Wed, 9 Jan 2019 18:07:19 +0100
Pierre Morel <pmorel@...ux.ibm.com> wrote:

> On 09/01/2019 16:37, Alex Williamson wrote:
> > On Wed,  9 Jan 2019 13:41:53 +0100
> > Pierre Morel <pmorel@...ux.ibm.com> wrote:
> >   
> >> We add a new flag, VFIO_IOMMU_INFO_CAPABILITIES, inside the
> >> vfio_iommu_type1_info to specify the support for capabilities.
> >>
> >> We add a new capability, with id VFIO_IOMMU_INFO_CAP_DMA
> >> in the capability list of the VFIO_IOMMU_GET_INFO ioctl.
> >>
> >> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
> >> ---
> >>   include/uapi/linux/vfio.h | 9 +++++++++
> >>   1 file changed, 9 insertions(+)
> >>
> >> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >> index 8131028..54c4fcb 100644
> >> --- a/include/uapi/linux/vfio.h
> >> +++ b/include/uapi/linux/vfio.h
> >> @@ -669,6 +669,15 @@ struct vfio_iommu_type1_info {
> >>   	__u32	flags;
> >>   #define VFIO_IOMMU_INFO_PGSIZES (1 << 0)	/* supported page sizes info */
> >>   	__u64	iova_pgsizes;		/* Bitmap of supported page sizes */
> >> +#define VFIO_IOMMU_INFO_CAPABILITIES (1 << 1)  /* support capabilities info */
> >> +	__u64   cap_offset;     /* Offset within info struct of first cap */
> >> +};
> >> +
> >> +#define VFIO_IOMMU_INFO_CAP_DMA 1
> >> +struct vfio_iommu_cap_dma {
> >> +	struct vfio_info_cap_header header;
> >> +	__u64   dma_start;
> >> +	__u64   dma_end;
> >>   };
> >>   
> >>   #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)  
> > 
> > Unfortunately for most systems, a simple start and end is not really
> > sufficient to describe the available IOVA space, there are often
> > reserved regions intermixed, so this is not really a complete
> > solution.  Shameer tried to solve this last year[1] but we ran into a
> > road block that Intel IGD devices impose a reserved range of IOVA
> > spaces reported to the user that conflict with existing assignment of
> > this device and we haven't figured out yet how to be more selective of
> > the enforcement of those reserved ranges.  Thanks,
> > 
> > Alex
> > 
> > [1] https://lkml.org/lkml/2018/4/18/293
> >   
> 
> I understand that some architecture may be more complex and have special 
> needs.
> However the IOMMU geometry is a constant for all IOMMU devices and
> is reported by the geometry in the iommu operations.
> 
> This makes the IOMMU geometry a special case.

I'm not so sure that the geometry is a constant for all IOMMU devices,
nor am I sure how if that were true and it's part of an in-kernel
interface that it automatically qualifies it as the right way to expose
it to userspace.  The fact that we have a reserved region interface to
augment a basic contiguous range suggests it's known to be insufficient
even for in-kernel use.

> It is also a special case because it is an inclusive description of 
> available memory, to oppose to the exclusive description given by the 
> windows.

Geometry doesn't really have anything to do with available memory, it's
the minimum and maximum IOVA aperture.  Shameer's proposal gave us an
IOVA list, which is based on the IOMMU geometry, from which it excludes
various reserved ranges.  So if you have a less complex architecture,
you might only have one entry in the list, which gives you the start
and end of the base geometry.  Move complex architectures might have
more entries, but the geometry can still be deduced from the absolute
highest and lowest addresses within the list.  Therefore a basic
geometry capability is automatically redundant to the interface that's
already been proposed.

> Isn't it possible to separate the IOMMU geometry, which is really 
> related to the IOMMU chip, from other windows exclusion related to the 
> system memory mapping?

Why would we ever have both given the description above?

> Retrieving the IOMMU geometry is very important for us because the 
> driver inside the guest must get it and program the IOMMU based on these 
> values.

So you have motivation to help move the IOVA list proposal forward,
or some equally inclusive proposal that isn't just a stop-gap ;)
Thanks,

Alex