linux-kernel - Re: [PATCH] KVM: arm64: Restrict IPA size to maximum 48 bits on 4K and 16K page size

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Date:   Wed, 11 Aug 2021 10:36:37 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Restrict IPA size to maximum 48 bits on 4K and 16K page size

On Wed, 11 Aug 2021 05:52:29 +0100,
Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
> Even though ID_AA64MMFR0.PARANGE reports 52 bit PA size support, it cannot
> be enabled as guest IPA size on 4K or 16K page size configurations. Hence
> kvm_ipa_limit must be restricted to 48 bits. This change achieves required
> IPA capping.
> 
> Before the commit c9b69a0cf0b4 ("KVM: arm64: Don't constrain maximum IPA
> size based on host configuration"), the problem here would have been just
> latent via PHYS_MASK_SHIFT (which earlier in turn capped kvm_ipa_limit),
> which remains capped at 48 bits on 4K and 16K configs.
> 
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: James Morse <james.morse@....com>
> Cc: Alexandru Elisei <alexandru.elisei@....com>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: kvmarm@...ts.cs.columbia.edu
> Cc: linux-kernel@...r.kernel.org
> Fixes: c9b69a0cf0b4 ("KVM: arm64: Don't constrain maximum IPA size based on host configuration")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
> This applies on v5.14-rc5
> 
>  arch/arm64/kvm/reset.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 20588220fe66..e66974c4b9d3 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -337,6 +337,15 @@ int kvm_set_ipa_limit(void)
>  		return -EINVAL;
>  	}
>  
> +	/*
> +	 * IPA size beyond 48 bits could not be supported
> +	 * on either 4K or 16K page size. Hence let's cap
> +	 * it to 48 bits, in case it's reported as larger
> +	 * on the system.
> +	 */
> +	if (!IS_ENABLED(CONFIG_ARM64_64K_PAGES))

As per our earlier discussion, please use (PAGE_SIZE != SZ_64K)
instead. This is in keeping with the rest of the file.

> +		parange = min(parange, (unsigned int)ID_AA64MMFR0_PARANGE_48);
> +

Also, please move it next to the point where we assign parange.

>  	kvm_ipa_limit = id_aa64mmfr0_parange_to_phys_shift(parange);
>  	kvm_info("IPA Size Limit: %d bits%s\n", kvm_ipa_limit,
>  		 ((kvm_ipa_limit < KVM_PHYS_SHIFT) ?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.