Re: [RFC PATCH 22/30] iommu: Bind/unbind tasks to/from devices

From: Joerg Roedel <joro@8bytes.org>
To: Jean-Philippe Brucker <Jean-Philippe.Brucker@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Shanker Donthineni <shankerd@qti.qualcomm.com>,
	kvm@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	Sinan Kaya <okaya@qti.qualcomm.com>,
	Will Deacon <will.deacon@arm.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Harv Abdulhamid <harba@qti.qualcomm.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	iommu@lists.linux-foundation.org, linux-pci@vger.kernel.org,
	Bjorn Helgaas <bhelgaas@google.com>,
	David Woodhouse <dwmw2@infradead.org>,
	linux-arm-kernel@lists.infradead.org,
	Nate Watterson <nwatters@qti.qualcomm.com>
Subject: Re: [RFC PATCH 22/30] iommu: Bind/unbind tasks to/from devices
Date: Wed, 22 Mar 2017 16:36:10 +0100	[thread overview]
Message-ID: <20170322153610.GC7266@8bytes.org> (raw)
In-Reply-To: <20170303183958.GA15146@e106794-lin.localdomain>

On Fri, Mar 03, 2017 at 06:39:58PM +0000, Jean-Philippe Brucker wrote:
> Yes, it would be nice to have a common PASID allocator. But I don't
> think that a system-wide PASID space is workable for us. At the moment
> systems might have a few identical devices all supporting 20 bits of
> PASID. But consider the case where one odd device can only handle four
> address spaces, and supports a maximum of two PASID bits. We'd quickly
> run out of PASIDs to hand to such devices, even though we could easily
> have one PASID space per endpoint (from a quick glance at the specs, I
> assume that both Intel and AMD IOMMUs offer one PASID table per RID.)

But that shouldn't be a problem if we allocate PASIDs top-down (meaning
starting from the biggest value supported by a given device), right?

Then we can satisfy the devices with 16 or 20 bit PASIDs and still have
the 2-bit PASIDs free for the devices that need it.

	Joerg

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