linux-kernel - Re: [kvmtool test PATCH 24/24] kvmtool: arm: Add support for creating VM with PA size

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Date:   Thu, 5 Jul 2018 08:51:19 +0100
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <cdall@...nel.org>,
        kvm-devel <kvm@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Punit Agrawal <punit.agrawal@....com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        kvmarm@...ts.cs.columbia.edu,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [kvmtool test PATCH 24/24] kvmtool: arm: Add support for creating
 VM with PA size

On 4 July 2018 at 16:51, Will Deacon <will.deacon@....com> wrote:
> On Wed, Jul 04, 2018 at 03:41:18PM +0100, Marc Zyngier wrote:
>> Not really. Let's say I want my IPA space split in two: memory covers
>> the low 47 bit, and I want MMIO spanning the top 47 bit. With your
>> scheme, you'd end-up with a 47bit IPA space, while you really want 48
>> bits (MMIO space implemented by userspace isn't registered to the
>> kernel).
>
> That still sounds quite niche for a VM. Does QEMU do that?

Not at 47 bits, but we have RAM up to the 256GB mark, and
MMIO above that (including a large PCI window), so the general
arrangement of having the top end of the IPA space not
necessarily be things we've told the kernel about definitely
exists.

thanks
-- PMM