RE: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

From: Anup Patel <Anup.Patel@wdc.com>
To: Palmer Dabbelt <palmer@sifive.com>, "will@kernel.org" <will@kernel.org>
Cc: "aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>,
	Arnd Bergmann <arnd@arndb.de>,
	"julien.thierry@arm.com" <julien.thierry@arm.com>,
	"marc.zyngier@arm.com" <marc.zyngier@arm.com>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"suzuki.poulose@arm.com" <suzuki.poulose@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"rppt@linux.ibm.com" <rppt@linux.ibm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Atish Patra <Atish.Patra@wdc.com>,
	"julien.grall@arm.com" <julien.grall@arm.com>,
	"guoren@kernel.org" <guoren@kernel.org>,
	"gary@garyguo.net" <gary@garyguo.net>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	"christoffer.dall@arm.com" <christoffer.dall@arm.com>,
	"james.morse@arm.com" <james.morse@arm.com>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file
Date: Sun, 15 Sep 2019 05:03:38 +0000	[thread overview]
Message-ID: <MN2PR04MB606117F2AC47385EF23D267D8D8D0@MN2PR04MB6061.namprd04.prod.outlook.com> (raw)
In-Reply-To: <mhng-166dcd4f-9483-4aab-a83a-914d70ddb5a4@palmer-si-x1e>


> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org <linux-kernel-
> owner@vger.kernel.org> On Behalf Of Palmer Dabbelt
> Sent: Saturday, September 14, 2019 7:31 PM
> To: will@kernel.org
> Cc: guoren@kernel.org; Will Deacon <will.deacon@arm.com>;
> julien.thierry@arm.com; aou@eecs.berkeley.edu; james.morse@arm.com;
> Arnd Bergmann <arnd@arndb.de>; suzuki.poulose@arm.com;
> marc.zyngier@arm.com; catalin.marinas@arm.com; Anup Patel
> <Anup.Patel@wdc.com>; linux-kernel@vger.kernel.org;
> rppt@linux.ibm.com; Christoph Hellwig <hch@infradead.org>; Atish Patra
> <Atish.Patra@wdc.com>; julien.grall@arm.com; gary@garyguo.net; Paul
> Walmsley <paul.walmsley@sifive.com>; christoffer.dall@arm.com; linux-
> riscv@lists.infradead.org; kvmarm@lists.cs.columbia.edu; linux-arm-
> kernel@lists.infradead.org; iommu@lists.linux-foundation.org
> Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
> separate file
> 
> On Thu, 12 Sep 2019 07:02:56 PDT (-0700), will@kernel.org wrote:
> > On Sun, Sep 08, 2019 at 07:52:55AM +0800, Guo Ren wrote:
> >> On Mon, Jun 24, 2019 at 6:40 PM Will Deacon <will@kernel.org> wrote:
> >> > > I'll keep my system use the same ASID for SMP + IOMMU :P
> >> >
> >> > You will want a separate allocator for that:
> >> >
> >> > https://lkml.kernel.org/r/20190610184714.6786-2-jean-philippe.bruck
> >> > er@arm.com
> >>
> >> Yes, it is hard to maintain ASID between IOMMU and CPUMMU or
> >> different system, because it's difficult to synchronize the IO_ASID
> >> when the CPU ASID is rollover.
> >> But we could still use hardware broadcast TLB invalidation
> >> instruction to uniformly manage the ASID and IO_ASID, or OTHER_ASID in
> our IOMMU.
> >
> > That's probably a bad idea, because you'll likely stall execution on
> > the CPU until the IOTLB has completed invalidation. In the case of
> > ATS, I think an endpoint ATC is permitted to take over a minute to
> > respond. In reality, I suspect the worst you'll ever see would be in
> > the msec range, but that's still an unacceptable period of time to hold a
> CPU.
> >
> >> Welcome to join our disscusion:
> >> "Introduce an implementation of IOMMU in linux-riscv"
> >> 9 Sep 2019, 10:45 Jade-room-I&II (Corinthia Hotel Lisbon) RISC-V MC
> >
> > I attended this session, but it unfortunately raised many more
> > questions than it answered.
> 
> Ya, we're a long way from figuring this out.

For everyone's reference, here is our first attempt at RISC-V ASID allocator:
http://archive.lwn.net:8080/linux-kernel/20190329045111.14040-1-anup.patel@wdc.com/T/#u

Regards,
Anup
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