On Tue, 27 Jun 2023 17:36:16 +0100, Oliver Upton oliver.upton@linux.dev wrote:
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.
Awesome, thanks.
(/me goes back to encoding FGT handling by hand... :-/)
M.