On 08/09/17 10:05, gengdongjiu wrote: > Marc, > Thanks for reply. > > On 2017/9/8 16:21, Marc Zyngier wrote: >>> 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...
