linux-kernel - Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM

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Date:   Tue, 18 Sep 2018 02:55:05 +0100
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, kvm-devel <kvm@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <cdall@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Dave P Martin <dave.martin@....com>, julien.grall@....com,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address
 size for VM

On 17 September 2018 at 11:41, Suzuki K Poulose <suzuki.poulose@....com> wrote:
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -122,6 +122,14 @@ the default trap & emulate implementation (which changes the virtual
>  memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
>  flag KVM_VM_MIPS_VZ.
>
> +To configure the physical address space size for a VM (IPA size) on arm64,
> +check KVM_CAP_ARM_VM_PHYS_SHIFT (which returns the maximum limit for the
> +IPA shift) and use KVM_VM_TYPE_ARM_PHYS_SHIFT(PHYS_SHIFT). Bits[7-0] of the
> +machine type has been reserved for specifying the PHYS_SHIFT.
> +The supported range is [32...IPA_LIMIT], where IPA_LIMIT could be
> +identified by checking KVM_CAP_ARM_VM_PHYS_SHIFT. For backward compatibility
> +a value of 0 selects 40bits.
> +

Given this as the API documentation, I don't think I could figure out
what I as a userspace user of it need to do without looking at the
kernel code. Could I ask you to expand it so that it is a bit less
terse and a bit more detailed? (For instance, what is a PHYS_SHIFT
and why do I have to specify it rather than just telling the kernel
I want a 48 bit guest address space?)

thanks
-- PMM