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Subject: Re: [PATCH v4 3/6] arm64/kvm: add a userspace option to enable
 pointer authentication
To:     Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc:     LAK <linux-arm-kernel@lists.infradead.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        kvmarm@lists.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
        Dave Martin <Dave.Martin@arm.com>, linux-kernel@vger.kernel.org
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From:   James Morse <james.morse@arm.com>
Message-ID: <3936d9fd-1e3f-0437-e866-edf2d9cbf797@arm.com>
Date:   Thu, 31 Jan 2019 16:19:35 +0000
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Hi Amit,

On 09/01/2019 10:13, Amit Daniel Kachhap wrote:
> On Sat, Jan 5, 2019 at 12:05 AM James Morse <james.morse@arm.com> wrote:
>> On 18/12/2018 07:56, Amit Daniel Kachhap wrote:
>>> This feature will allow the KVM guest to allow the handling of
>>> pointer authentication instructions or to treat them as undefined
>>> if not set. It uses the existing vcpu API KVM_ARM_VCPU_INIT to
>>> supply this parameter instead of creating a new API.
>>>
>>> A new register is not created to pass this parameter via
>>> SET/GET_ONE_REG interface as just a flag (KVM_ARM_VCPU_PTRAUTH)
>>> supplied is enough to select this feature.
>>
>> What is the motivation for doing this? Doesn't ptrauth 'just' need turning on?
>> It doesn't need additional setup to be useable, or rely on some qemu support to
>> work properly. There isn't any hidden state that can't be migrated in the usual way.
>> Is it just because we don't want to commit to the ABI?

> This allows migration of guest to non pointer authenticated supported
> systems and hides the extra ptrauth registers.

The MIDR already has to match, so the hardware must be the same. I guess this
lets us hide the new feature so old-guests can migrate to a new-kernel without a
write to the id registers failing.


> Basically this suggestion was given by Christoffer
> (https://lore.kernel.org/lkml/20180206123847.GY21802@cbox/).

Ah, Christoffer asked for it, that's reason enough!


Thanks,

James