Re: [PATCH v6 17/18] kvm: arm64: Limit the minimum number of page table levels

[Auger Eric]

Hi,
On 9/26/18 6:32 PM, Suzuki K Poulose wrote:
> Since we are about to remove the lower limit on the IPA size,
> make sure that we do not go to 1 level page table (e.g, with
> 32bit IPA on 64K host with concatenation) to avoid splitting
> the host PMD huge pages at stage2.
> 
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Christoffer Dall <cdall@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>

Thanks

Eric
> ---
> Change since v5:
>  - Cosmetic changes to the comment
>  - Remove unnecessary new line
> ---
>  arch/arm64/include/asm/stage2_pgtable.h |  7 ++++++-
>  arch/arm64/kvm/reset.c                  | 10 +++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/stage2_pgtable.h b/arch/arm64/include/asm/stage2_pgtable.h
> index c62fe118a898..2cce769ba4c6 100644
> --- a/arch/arm64/include/asm/stage2_pgtable.h
> +++ b/arch/arm64/include/asm/stage2_pgtable.h
> @@ -72,8 +72,13 @@
>  /*
>   * The number of PTRS across all concatenated stage2 tables given by the
>   * number of bits resolved at the initial level.
> + * If we force more levels than necessary, we may have (stage2_pgdir_shift > IPA),
> + * in which case, stage2_pgd_ptrs will have one entry.
>   */
> -#define __s2_pgd_ptrs(ipa, lvls)	(1 << ((ipa) - pt_levels_pgdir_shift((lvls))))
> +#define pgd_ptrs_shift(ipa, pgdir_shift)	\
> +	((ipa) > (pgdir_shift) ? ((ipa) - (pgdir_shift)) : 0)
> +#define __s2_pgd_ptrs(ipa, lvls)		\
> +	(1 << (pgd_ptrs_shift((ipa), pt_levels_pgdir_shift(lvls))))
>  #define __s2_pgd_size(ipa, lvls)	(__s2_pgd_ptrs((ipa), (lvls)) * sizeof(pgd_t))
>  
>  #define stage2_pgd_ptrs(kvm)		__s2_pgd_ptrs(kvm_phys_shift(kvm), kvm_stage2_levels(kvm))
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 96b3f50101bc..f156e45760bc 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -190,6 +190,7 @@ int kvm_arm_config_vm(struct kvm *kvm, unsigned long type)
>  {
>  	u64 vtcr = VTCR_EL2_FLAGS;
>  	u32 parange, phys_shift;
> +	u8 lvls;
>  
>  	if (type)
>  		return -EINVAL;
> @@ -203,7 +204,14 @@ int kvm_arm_config_vm(struct kvm *kvm, unsigned long type)
>  	if (phys_shift > KVM_PHYS_SHIFT)
>  		phys_shift = KVM_PHYS_SHIFT;
>  	vtcr |= VTCR_EL2_T0SZ(phys_shift);
> -	vtcr |= VTCR_EL2_LVLS_TO_SL0(stage2_pgtable_levels(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
>