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

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Message-ID: <20170908092155.4d90862b@why.wild-wind.fr.eu.org>
Date:   Fri, 8 Sep 2017 09:21:55 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     gengdongjiu <gengdongjiu@...wei.com>
Cc:     James Morse <james.morse@....com>, <christoffer.dall@...aro.org>,
        <vladimir.murzin@....com>, <rkrcmar@...hat.com>,
        <catalin.marinas@....com>, <shankerd@...eaurora.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <zhanghaibin7@...wei.com>,
        <huangshaoyu@...wei.com>
Subject: Re: [PATCH] arm64: KVM: VHE: reset PSTATE.UAO when switch to host

On Fri, 8 Sep 2017 15:19:21 +0800
gengdongjiu <gengdongjiu@...wei.com> wrote:

> 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.

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.

So how is PAN relevant in this context? What does it even mean?
If you have a ARMv8.0 behaviour, PAN doesn't even seem to *exist* at
EL2.

Or am I completely missing the point here?

	M.
-- 
Without deviation from the norm, progress is not possible.