linux-kernel - Re: [PATCH kernel v11 23/34] powerpc/iommu/powernv: Release replaced TCE

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Message-ID: <20150601070449.GN22789@voom.redhat.com>
Date:	Mon, 1 Jun 2015 17:04:49 +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>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel v11 23/34] powerpc/iommu/powernv: Release replaced
 TCE

On Fri, May 29, 2015 at 06:44:47PM +1000, Alexey Kardashevskiy wrote:
> At the moment writing new TCE value to the IOMMU table fails with EBUSY
> if there is a valid entry already. However PAPR specification allows
> the guest to write new TCE value without clearing it first.
> 
> Another problem this patch is addressing is the use of pool locks for
> external IOMMU users such as VFIO. The pool locks are to protect
> DMA page allocator rather than entries and since the host kernel does
> not control what pages are in use, there is no point in pool locks and
> exchange()+put_page(oldtce) is sufficient to avoid possible races.
> 
> This adds an exchange() callback to iommu_table_ops which does the same
> thing as set() plus it returns replaced TCE and DMA direction so
> the caller can release the pages afterwards. The exchange() receives
> a physical address unlike set() which receives linear mapping address;
> and returns a physical address as the clear() does.
> 
> This implements exchange() for P5IOC2/IODA/IODA2. This adds a requirement
> for a platform to have exchange() implemented in order to support VFIO.
> 
> This replaces iommu_tce_build() and iommu_clear_tce() with
> a single iommu_tce_xchg().
> 
> This makes sure that TCE permission bits are not set in TCE passed to
> IOMMU API as those are to be calculated by platform code from
> DMA direction.
> 
> This moves SetPageDirty() to the IOMMU code to make it work for both
> VFIO ioctl interface in in-kernel TCE acceleration (when it becomes
> available later).
> 
> 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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