linux-kernel - Re: [PATCH v2 10/18] arm64: KVM/debug: use EL1&0 stage 1 translation regime

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Date:   Wed, 15 Jan 2020 14:03:24 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, 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 10/18] arm64: KVM/debug: use EL1&0 stage 1 translation
 regime

On 2020-01-13 16:31, Andrew Murray wrote:
> On Sun, Dec 22, 2019 at 10:34:55AM +0000, Marc Zyngier wrote:
>> On Fri, 20 Dec 2019 14:30:17 +0000,
>> Andrew Murray <andrew.murray@....com> wrote:
>> >
>> > From: Sudeep Holla <sudeep.holla@....com>
>> >
>> > Now that we have all the save/restore mechanism in place, lets enable
>> > the translation regime used by buffer from EL2 stage 1 to EL1 stage 1
>> > on VHE systems.
>> >
>> > Signed-off-by: Sudeep Holla <sudeep.holla@....com>
>> > [ Reword commit, don't trap to EL2 ]
>> 
>> Not trapping to EL2 for the case where we don't allow SPE in the
>> guest is not acceptable.
>> 
>> > Signed-off-by: Andrew Murray <andrew.murray@....com>
>> > ---
>> >  arch/arm64/kvm/hyp/switch.c | 2 ++
>> >  1 file changed, 2 insertions(+)
>> >
>> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>> > index 67b7c160f65b..6c153b79829b 100644
>> > --- a/arch/arm64/kvm/hyp/switch.c
>> > +++ b/arch/arm64/kvm/hyp/switch.c
>> > @@ -100,6 +100,7 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
>> >
>> >  	write_sysreg(val, cpacr_el1);
>> >
>> > +	write_sysreg(vcpu->arch.mdcr_el2 | 3 << MDCR_EL2_E2PB_SHIFT, mdcr_el2);
>> >  	write_sysreg(kvm_get_hyp_vector(), vbar_el1);
>> >  }
>> >  NOKPROBE_SYMBOL(activate_traps_vhe);
>> > @@ -117,6 +118,7 @@ static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
>> >  		__activate_traps_fpsimd32(vcpu);
>> >  	}
>> >
>> > +	write_sysreg(vcpu->arch.mdcr_el2 | 3 << MDCR_EL2_E2PB_SHIFT, mdcr_el2);
>> 
>> There is a _MASK macro that can replace this '3', and is in keeping
>> with the rest of the code.
>> 
>> It still remains that it looks like the wrong place to do this, and
>> vcpu_load seems much better. Why should you write to mdcr_el2 on each
>> entry to the guest, since you know whether it has SPE enabled at the
>> point where it gets scheduled?
> 
> For nVHE, the only reason we'd want to change E2PB on entry/exit of 
> guest
> would be if the host is also using SPE. If the host is using SPE whilst
> the vcpu is 'loaded' but we're not in the guest, then host SPE could 
> raise
> an interrupt - we need the E2PB bits to allow access from EL1 (host).

My comment was of course for VHE. nVHE hardly makes use of load/put at 
all,
for obvious reasons.

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