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Date:   Wed, 13 Feb 2019 17:34:18 +0000
From:   Kristina Martsenko <>
To:     Amit Daniel Kachhap <>,
Cc:     Christoffer Dall <>,
        Marc Zyngier <>,
        Catalin Marinas <>,
        Will Deacon <>,
        Andrew Jones <>,
        Dave Martin <>,
        Ramana Radhakrishnan <>,,,
        Mark Rutland <>
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

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?



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