Re: [PATCH v5 2/5] arm64/kvm: preserve host HCR_EL2/MDCR_EL2 value

From: Amit Daniel Kachhap
Date: Thu Feb 14 2019 - 06:03:29 EST


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

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