Re: [PATCH v4 21/21] KVM: arm64: Don't constrain maximum IPA size based on host configuration

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Will Deacon <will@kernel.org>, kvmarm@lists.cs.columbia.edu
Cc: Gavin Shan <gshan@redhat.com>,
	Suzuki Poulose <suzuki.poulose@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Quentin Perret <qperret@google.com>,
	James Morse <james.morse@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	kernel-team@android.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 21/21] KVM: arm64: Don't constrain maximum IPA size based on host configuration
Date: Wed, 9 Sep 2020 15:53:21 +0100	[thread overview]
Message-ID: <05da664a-7abf-ce13-1201-61b86368933e@arm.com> (raw)
In-Reply-To: <20200907152344.12978-22-will@kernel.org>

Hi Will,

On 9/7/20 4:23 PM, Will Deacon wrote:
> Now that the guest stage-2 page-tables are managed independently from
> the host stage-1 page-tables, we can avoid constraining the IPA size
> based on the host and instead limit it only based on the PARange field
> of the ID_AA64MMFR0 register.
>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Quentin Perret <qperret@google.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kvm/reset.c | 38 +++++---------------------------------
>  1 file changed, 5 insertions(+), 33 deletions(-)
>
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index ee33875c5c2a..471ee9234e40 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -339,7 +339,7 @@ u32 get_kvm_ipa_limit(void)
>  
>  int kvm_set_ipa_limit(void)
>  {
> -	unsigned int ipa_max, pa_max, va_max, parange, tgran_2;
> +	unsigned int parange, tgran_2;
>  	u64 mmfr0;
>  
>  	mmfr0 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1);
> @@ -376,38 +376,10 @@ int kvm_set_ipa_limit(void)
>  		break;
>  	}
>  
> -	pa_max = id_aa64mmfr0_parange_to_phys_shift(parange);
> -
> -	/* Clamp the IPA limit to the PA size supported by the kernel */
> -	ipa_max = (pa_max > PHYS_MASK_SHIFT) ? PHYS_MASK_SHIFT : pa_max;
> -	/*
> -	 * Since our stage2 table is dependent on the stage1 page table code,
> -	 * we must always honor the following condition:
> -	 *
> -	 *  Number of levels in Stage1 >= Number of levels in Stage2.
> -	 *
> -	 * So clamp the ipa limit further down to limit the number of levels.
> -	 * Since we can concatenate upto 16 tables at entry level, we could
> -	 * go upto 4bits above the maximum VA addressable with the current
> -	 * number of levels.
> -	 */
> -	va_max = PGDIR_SHIFT + PAGE_SHIFT - 3;
> -	va_max += 4;
> -
> -	if (va_max < ipa_max)
> -		ipa_max = va_max;
> -
> -	/*
> -	 * If the final limit is lower than the real physical address
> -	 * limit of the CPUs, report the reason.
> -	 */
> -	if (ipa_max < pa_max)
> -		pr_info("kvm: Limiting the IPA size due to kernel %s Address limit\n",
> -			(va_max < pa_max) ? "Virtual" : "Physical");
> -
> -	WARN(ipa_max < KVM_PHYS_SHIFT,
> -	     "KVM IPA limit (%d bit) is smaller than default size\n", ipa_max);
> -	kvm_ipa_limit = ipa_max;
> +	kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
> +	WARN(kvm_ipa_limit < KVM_PHYS_SHIFT,
> +	     "KVM IPA limit (%d bit) is smaller than default size\n",
> +	     kvm_ipa_limit);
>  	kvm_info("IPA Size Limit: %dbits\n", kvm_ipa_limit);

Bikeshedding: the two messages are slightly inconsistent: "IPA limit" vs "IPA Size
Limit" and "%d bit" vs "%dbits. Might be worth using the exact wording to make it
painfully obvious that the messages refer to the same variable, kvm_ipa_limit.

Thanks,
Alex

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