Re: [RFC PATCH v2 21/26] KVM: arm64: Refactor kvm_arm_setup_stage2()

[Will Deacon]

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@xxxxxxxxxx>
> ---
>  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.

Will