linux-kernel - Re: [PATCH kernel v12 33/34] vfio: powerpc/spapr: Register memory and define IOMMU v2

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Date:	Tue, 9 Jun 2015 14:21:25 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	Alex Williamson <alex.williamson@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Gavin Shan <gwshan@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel v12 33/34] vfio: powerpc/spapr: Register memory
 and define IOMMU v2

On Fri, Jun 05, 2015 at 04:35:25PM +1000, Alexey Kardashevskiy wrote:
> The existing implementation accounts the whole DMA window in
> the locked_vm counter. This is going to be worse with multiple
> containers and huge DMA windows. Also, real-time accounting would requite
> additional tracking of accounted pages due to the page size difference -
> IOMMU uses 4K pages and system uses 4K or 64K pages.
> 
> Another issue is that actual pages pinning/unpinning happens on every
> DMA map/unmap request. This does not affect the performance much now as
> we spend way too much time now on switching context between
> guest/userspace/host but this will start to matter when we add in-kernel
> DMA map/unmap acceleration.
> 
> This introduces a new IOMMU type for SPAPR - VFIO_SPAPR_TCE_v2_IOMMU.
> New IOMMU deprecates VFIO_IOMMU_ENABLE/VFIO_IOMMU_DISABLE and introduces
> 2 new ioctls to register/unregister DMA memory -
> VFIO_IOMMU_SPAPR_REGISTER_MEMORY and VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY -
> which receive user space address and size of a memory region which
> needs to be pinned/unpinned and counted in locked_vm.
> New IOMMU splits physical pages pinning and TCE table update
> into 2 different operations. It requires:
> 1) guest pages to be registered first
> 2) consequent map/unmap requests to work only with pre-registered memory.
> For the default single window case this means that the entire guest
> (instead of 2GB) needs to be pinned before using VFIO.
> When a huge DMA window is added, no additional pinning will be
> required, otherwise it would be guest RAM + 2GB.
> 
> The new memory registration ioctls are not supported by
> VFIO_SPAPR_TCE_IOMMU. Dynamic DMA window and in-kernel acceleration
> will require memory to be preregistered in order to work.
> 
> The accounting is done per the user process.
> 
> This advertises v2 SPAPR TCE IOMMU and restricts what the userspace
> can do with v1 or v2 IOMMUs.
> 
> In order to support memory pre-registration, we need a way to track
> the use of every registered memory region and only allow unregistration
> if a region is not in use anymore. So we need a way to tell from what
> region the just cleared TCE was from.
> 
> This adds a userspace view of the TCE table into iommu_table struct.
> It contains userspace address, one per TCE entry. The table is only
> allocated when the ownership over an IOMMU group is taken which means
> it is only used from outside of the powernv code (such as VFIO).
> 
> As v2 IOMMU supports IODA2 and pre-IODA2 IOMMUs (which do not support
> DDW API), this creates a default DMA window for IODA2 for consistency.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
> [aw: for the vfio related changes]
> Acked-by: Alex Williamson <alex.williamson@...hat.com>

Reviewed-by: David Gibson <david@...son.dropbear.id.au>

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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