linux-kernel - Re: [PATCH v2 02/18] arm64: KVM: reset E2PB correctly in MDCR

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Message-ID: <20191224102949.GD42593@e119886-lin.cambridge.arm.com>
Date:   Tue, 24 Dec 2019 10:29:50 +0000
From:   Andrew Murray <andrew.murray@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     will@...nel.org, Catalin Marinas <catalin.marinas@....com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sudeep Holla <sudeep.holla@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 02/18] arm64: KVM: reset E2PB correctly in MDCR_EL2
 when exiting the guest(VHE)

On Sat, Dec 21, 2019 at 01:12:14PM +0000, Marc Zyngier wrote:
> On Fri, 20 Dec 2019 14:30:09 +0000
> Andrew Murray <andrew.murray@....com> wrote:
> 
> > From: Sudeep Holla <sudeep.holla@....com>
> > 
> > On VHE systems, the reset value for MDCR_EL2.E2PB=b00 which defaults
> > to profiling buffer using the EL2 stage 1 translations. 
> 
> Does the reset value actually matter here? I don't see it being
> specific to VHE systems, and all we're trying to achieve is to restore
> the SPE configuration to a state where it can be used by the host.
> 
> > However if the
> > guest are allowed to use profiling buffers changing E2PB settings, we
> 
> How can the guest be allowed to change E2PB settings? Or do you mean
> here that allowing the guest to use SPE will mandate changes of the
> E2PB settings, and that we'd better restore the hypervisor state once
> we exit?
> 
> > need to ensure we resume back MDCR_EL2.E2PB=b00. Currently we just
> > do bitwise '&' with MDCR_EL2_E2PB_MASK which will retain the value.
> > 
> > So fix it by clearing all the bits in E2PB.
> > 
> > Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> > Signed-off-by: Andrew Murray <andrew.murray@....com>
> > ---
> >  arch/arm64/kvm/hyp/switch.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 72fbbd86eb5e..250f13910882 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -228,9 +228,7 @@ void deactivate_traps_vhe_put(void)
> >  {
> >  	u64 mdcr_el2 = read_sysreg(mdcr_el2);
> >  
> > -	mdcr_el2 &= MDCR_EL2_HPMN_MASK |
> > -		    MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT |
> > -		    MDCR_EL2_TPMS;
> > +	mdcr_el2 &= MDCR_EL2_HPMN_MASK | MDCR_EL2_TPMS;
> >  
> >  	write_sysreg(mdcr_el2, mdcr_el2);
> >  
> 
> I'm OK with this change, but I believe the commit message could use
> some tidying up.

No problem, I'll update the commit message.

Thanks,

Andrew Murray

> 
> Thanks,
> 
> 	M.
> -- 
> Jazz is not dead. It just smells funny...