linux-kernel - Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM

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Message-ID: <8392ecb1-50ce-c448-54f7-bf3021ae508b@arm.com>
Date:   Mon, 2 Jul 2018 14:31:13 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, james.morse@....com,
        cdall@...nel.org, eric.auger@...hat.com, julien.grall@....com,
        will.deacon@....com, catalin.marinas@....com,
        punit.agrawal@....com, qemu-devel@...gnu.org,
        Peter Maydel <peter.maydell@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical
 address size for VM

On 02/07/18 14:13, Marc Zyngier wrote:
> On 29/06/18 12:15, Suzuki K Poulose wrote:
>> Allow specifying the physical address size for a new VM via
>> the kvm_type argument for KVM_CREATE_VM ioctl. This allows
>> us to finalise the stage2 page table format as early as possible
>> and hence perform the right checks on the memory slots without
>> complication. The size is encoded as Log2(PA_Size) in the bits[7:0]
>> of the type field and can encode more information in the future if
>> required. The IPA size is still capped at 40bits.
> 
> Can't we relax this? There is no technical reason (AFAICS) not to allow
> going down to 36bit IPA if the user has requested it.

Sure, we can.

> 
> If we run on a 36bit IPA system, the default would fail. But if the user
> specified "please give me a 36bit IPA VM", we could satisfy that
> requirement and allow them to run their stupidly small guest!

Absolutely. I will fix this in the next version.

Cheers
Suzuki