[PATCH v4 17/40] KVM: arm64: Move userspace system registers into separate function

On Mon, Feb 19, 2018 at 05:21:17PM +0000, Julien Grall wrote:

Hi Christoffer,

On 15/02/18 21:03, Christoffer Dall wrote:

quoted

There's a semantic difference between the EL1 registers that control
operation of a kernel running in EL1 and EL1 registers that only control
userspace execution in EL0.  Since we can defer saving/restoring the
latter, move them into their own function.

ACTLR_EL1 is not used by a VHE host, so we can move this register into
the EL1 state which is not saved/restored for a VHE host.

Looking at D10.2.1 (ARM DDI 0487C.a), the statement regarding the use of
ACTLR_EL1 seems to be less strong than what you state here. It looks like it
would be possible to have hardware where ACTLR_EL1 would still have an
effect on host EL0. I also read the comments on the version 2 of this patch
but I wasn't able to find what I missing.

As Marc points out, I'll add a reference to the spec that says that Arm
recommends ACTLR_EL1 not having an effect on VHE host EL0.  While
potentially possible, it's unlikely, and we can fix that later if anyone
builds a platform with VHE that we care about that uses ACTLR_EL1 for
VHE host EL0.

Thanks,
-Christoffer