linux-kernel - Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

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Date:   Fri, 8 Sep 2017 15:19:21 +0800
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>
CC:     "christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
        "vladimir.murzin@....com" <vladimir.murzin@....com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "shankerd@...eaurora.org" <shankerd@...eaurora.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Zhanghaibin (Euler)" <zhanghaibin7@...wei.com>,
        Huangshaoyu <huangshaoyu@...wei.com>
Subject: Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

On 2017/9/7 23:23, Marc Zyngier wrote:
> On 07/09/17 16:03, gengdongjiu wrote:
>>> On 07/09/17 12:49, gengdongjiu wrote:
>>>>
[...]

> 
> I really cannot think of a good reason why we'd want to do that. Playing
> with set_fs() is almost universally wrong, and I'm certainly going to
> oppose to any change in that area unless the code that calls set_fs()
> has been made public and properly reviewed. Until then, UAO/PAN will
> stay as they are unless you prove that our current code is wrong.

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.

> 
> 	M.
>