Re: [PATCH 1/4] KVM: arm64: Ignore 'kvm-arm.mode=protected' when using VHE

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: kvmarm@lists.cs.columbia.edu, Marc Zyngier <maz@kernel.org>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Fuad Tabba <tabba@google.com>,
	Quentin Perret <qperret@google.com>,
	Sean Christopherson <seanjc@google.com>,
	David Brazdil <dbrazdil@google.com>,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/4] KVM: arm64: Ignore 'kvm-arm.mode=protected' when using VHE
Date: Mon, 7 Jun 2021 20:28:18 +0100	[thread overview]
Message-ID: <20210607192818.GA7929@willie-the-truck> (raw)
In-Reply-To: <20210604140117.GA69333@C02TD0UTHF1T.local>

On Fri, Jun 04, 2021 at 03:01:17PM +0100, Mark Rutland wrote:
> On Thu, Jun 03, 2021 at 07:33:44PM +0100, Will Deacon wrote:
> > Ignore 'kvm-arm.mode=protected' when using VHE so that kvm_get_mode()
> > only returns KVM_MODE_PROTECTED on systems where the feature is available.
> 
> IIUC, since the introduction of the idreg-override code, and the
> mutate_to_vhe stuff, passing 'kvm-arm.mode=protected' should make the
> kernel stick to EL1, right? So this should only affect M1 (or other HW
> with a similar impediment).

It's not just about the M1, unfortunately. You can boot with:

	"kvm-arm.mode=protected id_aa64mmfr1.vh=1"

which will force VHE mode, so we should fail protected mode in that case.

> One minor comment below; otherwise:
> 
> Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks, I'll keep the tag but please yell if you want me to drop it.

> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index efed2830d141..dc1f2e747828 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1773,15 +1773,7 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
> >  #ifdef CONFIG_KVM
> >  static bool is_kvm_protected_mode(const struct arm64_cpu_capabilities *entry, int __unused)
> >  {
> > -	if (kvm_get_mode() != KVM_MODE_PROTECTED)
> > -		return false;
> > -
> > -	if (is_kernel_in_hyp_mode()) {
> > -		pr_warn("Protected KVM not available with VHE\n");
> > -		return false;
> > -	}
> > -
> > -	return true;
> > +	return kvm_get_mode() == KVM_MODE_PROTECTED;
> >  }
> >  #endif /* CONFIG_KVM */
> >  
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 1cb39c0803a4..8d5e23198dfd 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -2121,7 +2121,11 @@ static int __init early_kvm_mode_cfg(char *arg)
> >  		return -EINVAL;
> >  
> >  	if (strcmp(arg, "protected") == 0) {
> > -		kvm_mode = KVM_MODE_PROTECTED;
> > +		if (!is_kernel_in_hyp_mode())
> > +			kvm_mode = KVM_MODE_PROTECTED;
> > +		else
> > +			pr_warn_once("Protected KVM not available with VHE\n");
> 
> ... assuming this is only for M1, it might be better to say:
> 
> 	Protected KVM not available on this hardware
> 
> ... since that doesn't suggest that other VHE-capable HW is also not
> PKVM-capable.

I'm just moving the existing string here, but as above, it's not M1
specific.

Will