[PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

On 08/09/17 10:05, gengdongjiu wrote:

Marc,
   Thanks for reply.

On 2017/9/8 16:21, Marc Zyngier wrote:

quoted

quoted

Marc,

sorry I have another question for the PAN.

In the non-VHE mode, The host kernel is running in the EL1. Before
host kernel enter guest, host OS will call 'HVC' instruction to do
the world-switch, and the pstate.PAN will be saved into the SPSR_EL2.
When world-switch back to host kernel from EL2, it will call 'eret'
instruction to EL1 host, this 'eret' instruction will restore the
SPSR_EL2 to the PSTATE. so the PSTATE.PAN will be restored.

For the Non-VHE mode, in the EL2 where mainly have word-switch code,
do you think it needs to reset the PSTATE.PAN? From the spec, it does
not provide SCTLR_EL2.SPAN bit for non-VHE mode, so reset the
PSTATE.PAN does not sure whether it is needed or whether affects the
performance. If you think it is needed for El2 in Non-VHE mode,
moving the reset PSTATE.PAN to the exception entry to EL2 may be
better, such as "el1_sync", because host can also call 'hvc'
instruction without guest running.

So let's see if I correctly understand your question:

You're worried that we don't set/reset PSTATE.PAN at EL2 in non-VHE?
In non-VHE, there is no user-space mapping that is present at the
same time as the hypervisor mappings. Actually, we hardly have any
mapping other than the HYP text/data and the vcpu/vm structures.

Not that meaning.
there are two meanings:

In short, we should not set PAN for El2 in non-VHE; If you think we should, current code does not cover all scenarios.


1. In the current mainline code it sets the PSTATE.PAN at EL2 in non-VHE. As you said,
in non-VHE, there is no user-space mapping that is present at the same time as the
hypervisor mappings, so I think it may not need to set both for EL1 and El2 in non-VHE,
but current code sets it. As you see[1], the code does not check VHE.

2. Conversely, in non-VHE, if you think we should set PAN in the EL2,

It is not about what I think. It is about what the architecture gives you.

There cannot be any userspace mapping at EL2 when non-VHE, so there
cannot be any valid PAN setting. I repeat: there is not such thing as
PAN at EL2 when HCR_EL2.E2H==0. This bit *has no effect*. Just read the
documentation (ARM DDI 0487B.a, D4.4.2).

If you're going to change this kind of code, please start by
understanding the architecture.

	M.
-- 
Jazz is not dead. It just smells funny...