Re: [PATCH] KVM: arm64: timers: Use CNTHCTL_EL2 when setting non-CNTKCTL_EL1 bits

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Jun 27, 2023 at 03:05:57PM +0100, Marc Zyngier wrote:
> It recently appeared that, whien running VHE, there is a notable
> difference between using CNTKCTL_EL1 and CNTHCTL_EL2, despite what
> the architecture documents:
> 
> - When accessed from EL2, bits [19:18] and [16:10] same bits have
>   the same assignment as CNTHCTL_EL2
> - When accessed from EL1, bits [19:18] and [16:10] are RES0
> 
> It is all OK, until you factor in NV, where the EL2 guest runs at EL1.
> In this configuration, CNTKCTL_EL11 doesn't trap, nor ends up in
> the VNCR page. This means that any write from the guest affecting
> CNTHCTL_EL2 using CNTKCTL_EL1 ends up losing some state. Not good.
> 
> The fix it obvious: don't use CNTKCTL_EL1 if you want to change bits
> that are not part of the EL1 definition of CNTKCTL_EL1, and use
> CNTHCTL_EL2 instead. This doesn't change anything for a bare-metal OS,
> and fixes it when running under NV. The NV hypervisor will itself
> have to work harder to merge the two accessors.
> 
> Note that there is a pending update to the architecture to address
> this issue by making the affected bits UNKNOWN when CNTKCTL_EL1 is
> user from EL2 with VHE enabled.
> 
> Fixes: c605ee245097 ("KVM: arm64: timers: Allow physical offset without CNTPOFF_EL2")
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org # v6.4

Looks good. I'll probably open a fixes branch around -rc1 and pick this
patch up then.

-- 
Thanks,
Oliver