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

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Message-ID: <CAFEAcA_tphfZ_BKfAYuefStTipta0notyAR=NCnWFJ6nRfERpg@mail.gmail.com>
Date:   Tue, 18 Sep 2018 18:15:04 +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 18 September 2018 at 17:27, Suzuki K Poulose <Suzuki.Poulose@....com> wrote:
> ---
>
> "On arm64, the physical address size for a VM (IPA Size limit) is limited
> to 40bits by default. The limit can be configured if the host supports the
> extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use
> KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type
> identifier, where IPA_Bits is the maximum width of any physical
> address used by the VM. The IPA_Bits is encoded in bits[7-0] of the
> machine type identifier.
>
> e.g, to configure a guest to use 48bit physical address size :
>
>         vm_fd = ioctl(dev_fd, KVM_CREATE_VM, KVM_VM_TYPE_ARM_IPA_SIZE(48));
>
> The requested size (IPA_Bits) must be :
>   0 - Implies default 40bits (for backward compatibility)
>
>   or
>
>   N - Implies N bits, where N is a positive integer such that 32 <= N <=
> Host_IPA_Limit
>
> Host_IPA_Limit is the maximum possible value for IPA_Bits on the host and
> is dependent on the CPU capability and the kernel configuration. The limit
> can
> be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
> ioctl() at run-time.
>
> Please note that configuring the IPA size does not affect the capability
> exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects
> the guest to host physical address (stage2) translations setup by the host.
> "

Thanks, this is much clearer. The only bit I'm not sure about is that
last paragraph -- if I ask for a VM with a 48 bit address space why
don't we tell the guest that that's what it has ?

thanks
-- PMM