linux-kernel - Re: [PATCH 07/16] KVM: arm64: Wire MMIO guard hypercalls

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Message-ID: <87v94p1kds.wl-maz@kernel.org>
Date:   Sun, 01 Aug 2021 12:20:47 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        qperret@...gle.com, dbrazdil@...gle.com,
        Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        kernel-team@...roid.com
Subject: Re: [PATCH 07/16] KVM: arm64: Wire MMIO guard hypercalls

On Fri, 30 Jul 2021 14:11:03 +0100,
Will Deacon <will@...nel.org> wrote:
> 
> On Wed, Jul 28, 2021 at 11:47:20AM +0100, Marc Zyngier wrote:
> > On Tue, 27 Jul 2021 19:11:46 +0100,
> > Will Deacon <will@...nel.org> wrote:
> > > 
> > > On Thu, Jul 15, 2021 at 05:31:50PM +0100, Marc Zyngier wrote:
> > > > Plumb in the hypercall interface to allow a guest to discover,
> > > > enroll, map and unmap MMIO regions.
> > > > 
> > > > Signed-off-by: Marc Zyngier <maz@...nel.org>
> > > > ---
> > > >  arch/arm64/kvm/hypercalls.c | 20 ++++++++++++++++++++
> > > >  include/linux/arm-smccc.h   | 28 ++++++++++++++++++++++++++++
> > > >  2 files changed, 48 insertions(+)
> > > > 
> > > > diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> > > > index 30da78f72b3b..a3deeb907fdd 100644
> > > > --- a/arch/arm64/kvm/hypercalls.c
> > > > +++ b/arch/arm64/kvm/hypercalls.c
> > > > @@ -5,6 +5,7 @@
> > > >  #include <linux/kvm_host.h>
> > > >  
> > > >  #include <asm/kvm_emulate.h>
> > > > +#include <asm/kvm_mmu.h>
> > > >  
> > > >  #include <kvm/arm_hypercalls.h>
> > > >  #include <kvm/arm_psci.h>
> > > > @@ -129,10 +130,29 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> > > >  	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
> > > >  		val[0] = BIT(ARM_SMCCC_KVM_FUNC_FEATURES);
> > > >  		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_PTP);
> > > > +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_INFO);
> > > > +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_ENROLL);
> > > > +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_MAP);
> > > > +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_MMIO_GUARD_UNMAP);
> > > >  		break;
> > > >  	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
> > > >  		kvm_ptp_get_time(vcpu, val);
> > > >  		break;
> > > > +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_INFO_FUNC_ID:
> > > > +		val[0] = PAGE_SIZE;
> > > > +		break;
> > > 
> > > I get the nagging feeling that querying the stage-2 page-size outside of
> > > MMIO guard is going to be useful once we start looking at memory sharing,
> > > so perhaps rename this to something more generic?
> > 
> > At this stage, why not follow the architecture and simply expose it as
> > ID_AA64MMFR0_EL1.TGran{4,64,16}_2? That's exactly what it is for, and
> > we already check for this in KVM itself.
> 
> Nice, I hadn't thought of that. On reflection, though, I don't agree that
> it's "exactly what it is for" -- the ID register talks about the supported
> stage-2 page-sizes, whereas we want to advertise the one page size that
> we're currently using. In other words, it's important that we only ever
> populate one of the fields and I wonder if that could bite us in future
> somehow?

Either that, or we expose all the page sizes >= to that of the host
(using the fact that larger page sizes are multiples of the base one),
and use the guest's page size to work out the granularity. Which is
what NV does already.

> Up to you, you've definitely got a better feel for this than me.

I'll have a look. The "one size" version is dead easy.

> 
> > > > +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_ENROLL_FUNC_ID:
> > > > +		set_bit(KVM_ARCH_FLAG_MMIO_GUARD, &vcpu->kvm->arch.flags);
> > > > +		val[0] = SMCCC_RET_SUCCESS;
> > > > +		break;
> > > > +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_MAP_FUNC_ID:
> > > > +		if (kvm_install_ioguard_page(vcpu, vcpu_get_reg(vcpu, 1)))
> > > > +			val[0] = SMCCC_RET_SUCCESS;
> > > > +		break;
> > > > +	case ARM_SMCCC_VENDOR_HYP_KVM_MMIO_GUARD_UNMAP_FUNC_ID:
> > > > +		if (kvm_remove_ioguard_page(vcpu, vcpu_get_reg(vcpu, 1)))
> > > > +			val[0] = SMCCC_RET_SUCCESS;
> > > > +		break;
> > > 
> > > I think there's a slight discrepancy between MAP and UNMAP here in that
> > > calling UNMAP on something that hasn't been mapped will fail, whereas
> > > calling MAP on something that's already been mapped will succeed. I think
> > > that might mean you can't reason about the final state of the page if two
> > > vCPUs race to call these functions in some cases (and both succeed).
> > 
> > I'm not sure that's the expected behaviour for ioremap(), for example
> > (you can ioremap two portions of the same page successfully).
> 
> Hmm, good point. Does that mean we should be refcounting the stage-2?
> Otherwise if we do something like:
> 
> 	foo = ioremap(page, 0x100);
> 	bar = ioremap(page+0x100, 0x100);
> 	iounmap(foo);
> 
> then bar will break. Or did I miss something in the series?

No, I don't think you have. But I don't think we should implement this
refcounting in the hypervisor side. We really should do it guest side.

I'll have a look.

Thanks,

	M.

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