Re: [PATCH v5 07/12] KVM: arm64: Use LPA2 page-tables for stage2 and hyp stage1

From: Oliver Upton <oliver.upton@linux.dev>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	James Morse <james.morse@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
Subject: Re: [PATCH v5 07/12] KVM: arm64: Use LPA2 page-tables for stage2 and hyp stage1
Date: Tue, 21 Nov 2023 20:34:50 +0000	[thread overview]
Message-ID: <ZV0Uaoi-qBjX3j5i@linux.dev> (raw)
In-Reply-To: <20231116142931.1675485-8-ryan.roberts@arm.com>

On Thu, Nov 16, 2023 at 02:29:26PM +0000, Ryan Roberts wrote:
> Implement a simple policy whereby if the HW supports FEAT_LPA2 for the
> page size we are using, always use LPA2-style page-tables for stage 2
> and hyp stage 1 (assuming an nvhe hyp), regardless of the VMM-requested
> IPA size or HW-implemented PA size. When in use we can now support up to
> 52-bit IPA and PA sizes.
> 
> We use the previously created cpu feature to track whether LPA2 is
> supported for deciding whether to use the LPA2 or classic pte format.
> 
> Note that FEAT_LPA2 brings support for bigger block mappings (512GB with
> 4KB, 64GB with 16KB). We explicitly don't enable these in the library
> because stage2_apply_range() works on batch sizes of the largest used
> block mapping, and increasing the size of the batch would lead to soft
> lockups. See commit 5994bc9e05c2 ("KVM: arm64: Limit
> stage2_apply_range() batch size to largest block").
> 
> With the addition of LPA2 support in the hypervisor, the PA size
> supported by the HW must be capped with a runtime decision, rather than
> simply using a compile-time decision based on PA_BITS. For example, on a
> system that advertises 52 bit PA but does not support FEAT_LPA2, A 4KB
> or 16KB kernel compiled with LPA2 support must still limit the PA size
> to 48 bits.
> 
> Therefore, move the insertion of the PS field into TCR_EL2 out of
> __kvm_hyp_init assembly code and instead do it in cpu_prepare_hyp_mode()
> where the rest of TCR_EL2 is prepared. This allows us to figure out PS
> with kvm_get_parange(), which has the appropriate logic to ensure the
> above requirement. (and the PS field of VTCR_EL2 is already populated
> this way).
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  arch/arm64/include/asm/kvm_mmu.h     |  2 +-
>  arch/arm64/include/asm/kvm_pgtable.h | 47 +++++++++++++++++++++-------
>  arch/arm64/kvm/arm.c                 |  5 +++
>  arch/arm64/kvm/hyp/nvhe/hyp-init.S   |  4 ---
>  arch/arm64/kvm/hyp/pgtable.c         | 15 +++++++--
>  5 files changed, 54 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 31e8d7faed65..f4e4fcb35afc 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -340,7 +340,7 @@ static inline struct kvm *kvm_s2_mmu_to_kvm(struct kvm_s2_mmu *mmu)
>  	return container_of(mmu->arch, struct kvm, arch);
>  }
>  
> -#define kvm_lpa2_is_enabled()		false
> +#define kvm_lpa2_is_enabled()		system_supports_lpa2()

Can we use this predicate consistently throughout the KVM code? Looks
like the rest of this diff is using system_supports_lpa2() directly.

-- 
Thanks,
Oliver

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