linux-kernel - Re: [RFC PATCH v2 21/26] KVM: arm64: Refactor kvm_arm_setup

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Message-ID: <YBv/iEl/Bjlte+yq@google.com>
Date:   Thu, 4 Feb 2021 14:07:04 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, android-kvm@...gle.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>
Subject: Re: [RFC PATCH v2 21/26] KVM: arm64: Refactor kvm_arm_setup_stage2()

On Wednesday 03 Feb 2021 at 15:53:54 (+0000), Will Deacon wrote:
> On Fri, Jan 08, 2021 at 12:15:19PM +0000, Quentin Perret wrote:
> > In order to re-use some of the stage 2 setup at EL2, factor parts of
> > kvm_arm_setup_stage2() out into static inline functions.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > ---
> >  arch/arm64/include/asm/kvm_mmu.h | 48 ++++++++++++++++++++++++++++++++
> >  arch/arm64/kvm/reset.c           | 42 +++-------------------------
> >  2 files changed, 52 insertions(+), 38 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index 662f0415344e..83b4c5cf4768 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -280,6 +280,54 @@ static inline int kvm_write_guest_lock(struct kvm *kvm, gpa_t gpa,
> >  	return ret;
> >  }
> >  
> > +static inline u64 kvm_get_parange(u64 mmfr0)
> > +{
> > +	u64 parange = cpuid_feature_extract_unsigned_field(mmfr0,
> > +				ID_AA64MMFR0_PARANGE_SHIFT);
> > +	if (parange > ID_AA64MMFR0_PARANGE_MAX)
> > +		parange = ID_AA64MMFR0_PARANGE_MAX;
> > +
> > +	return parange;
> > +}
> > +
> > +/*
> > + * The VTCR value is common across all the physical CPUs on the system.
> > + * We use system wide sanitised values to fill in different fields,
> > + * except for Hardware Management of Access Flags. HA Flag is set
> > + * unconditionally on all CPUs, as it is safe to run with or without
> > + * the feature and the bit is RES0 on CPUs that don't support it.
> > + */
> > +static inline u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift)
> > +{
> > +	u64 vtcr = VTCR_EL2_FLAGS;
> > +	u8 lvls;
> > +
> > +	vtcr |= kvm_get_parange(mmfr0) << VTCR_EL2_PS_SHIFT;
> > +	vtcr |= VTCR_EL2_T0SZ(phys_shift);
> > +	/*
> > +	 * Use a minimum 2 level page table to prevent splitting
> > +	 * host PMD huge pages at stage2.
> > +	 */
> > +	lvls = stage2_pgtable_levels(phys_shift);
> > +	if (lvls < 2)
> > +		lvls = 2;
> > +	vtcr |= VTCR_EL2_LVLS_TO_SL0(lvls);
> > +
> > +	/*
> > +	 * Enable the Hardware Access Flag management, unconditionally
> > +	 * on all CPUs. The features is RES0 on CPUs without the support
> > +	 * and must be ignored by the CPUs.
> > +	 */
> > +	vtcr |= VTCR_EL2_HA;
> > +
> > +	/* Set the vmid bits */
> > +	vtcr |= (get_vmid_bits(mmfr1) == 16) ?
> > +		VTCR_EL2_VS_16BIT :
> > +		VTCR_EL2_VS_8BIT;
> > +
> > +	return vtcr;
> > +}
> 
> Although I think this is functionally fine, I think it's unusual to see
> large "static inline" functions like this in shared header files. One
> alternative approach would be to follow the example of
> kernel/locking/qspinlock_paravirt.h, where the header is guarded in such a
> way that is only ever included by kernel/locking/qspinlock.c and therefore
> doesn't need the "inline" at all. That separation really helps, I think.

Alternatively, I might be able to have an mmu.c file in the hyp/ folder,
and to compile it for both the host kernel and the EL2 obj as we do for
a few things already. Or maybe I'll just stick it in pgtable.c. Either
way, it'll add a function call, but I can't really see that having any
measurable impact, so we should be fine.

Cheers,
Quentin