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

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Message-ID: <1a2534b9-6d3b-caac-ff05-7795b7a2d725@arm.com>
Date:   Thu, 14 Feb 2019 16:33:21 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     Kristina Martsenko <kristina.martsenko@....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

Hi,

On 2/13/19 11:04 PM, Kristina Martsenko wrote:
> 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.

This patch is not directly related to pointer authentication but just a 
helper to optimize save/restore. In this way save may be avoided for 
each switch and only restore is done. Patch 3 does sets HCR_EL2 in VHE_RUN.
> 
> 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?
Now HCR_EL2 is modified during switch time and NHVE doesnt support 
ptrauth so [2] doesn't makes sense.

//Amit D
> 
> 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/
>