linux-kernel - Re: [PATCH v5 2/5] arm64/kvm: preserve host HCR_EL2/MDCR

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Message-ID: <8de32a89-4aa1-bffb-78dd-45feddc9216e@arm.com>
Date:   Wed, 13 Feb 2019 17:34:18 +0000
From:   Kristina Martsenko <kristina.martsenko@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Jones <drjones@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 2/5] arm64/kvm: preserve host HCR_EL2/MDCR_EL2 value

On 28/01/2019 06:58, Amit Daniel Kachhap wrote:
> When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
> is a constant value. This works today, as the host HCR_EL2 value is
> always the same, but this will get in the way of supporting extensions
> that require HCR_EL2 bits to be set conditionally for the host.
> 
> To allow such features to work without KVM having to explicitly handle
> every possible host feature combination, this patch has KVM save/restore
> the host HCR when switching to/from a guest HCR. The saving of the
> register is done once during cpu hypervisor initialization state and is
> just restored after switch from guest.

Why is this patch needed? I couldn't find anything in this series that
sets HCR_EL2 conditionally for the host. It seems like the kernel still
always sets it to HCR_HOST_VHE_FLAGS/HCR_HOST_NVHE_FLAGS.

Looking back at v2 of the userspace pointer auth series, it seems that
the API/APK bits were set conditionally [1], so this patch would have
been needed to preserve HCR_EL2. But as of v3 of that series, the bits
have been set unconditionally through HCR_HOST_NVHE_FLAGS [2].

Is there something else I've missed?

Thanks,
Kristina

[1] https://lore.kernel.org/linux-arm-kernel/20171127163806.31435-6-mark.rutland@arm.com/
[2] https://lore.kernel.org/linux-arm-kernel/20180417183735.56985-5-mark.rutland@arm.com/