linux-kernel - Re: [PATCH v4 2/8] KVM: arm64: Introduce pkvm_alloc_private_va

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Date:   Wed, 02 Mar 2022 07:46:35 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     will@...nel.org, qperret@...gle.com, tabba@...gle.com,
        surenb@...gle.com, kernel-team@...roid.com,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        Andrew Scull <ascull@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/8] KVM: arm64: Introduce pkvm_alloc_private_va_range()

On Fri, 25 Feb 2022 03:34:47 +0000,
Kalesh Singh <kaleshsingh@...gle.com> wrote:
> 
> pkvm_hyp_alloc_private_va_range() can be used to reserve private VA ranges
> in the pKVM nVHE hypervisor (). Also update __pkvm_create_private_mapping()
> to allow specifying an alignment for the private VA mapping.
> 
> These will be used to implement stack guard pages for pKVM nVHE hypervisor
> (in a subsequent patch in the series).
> 
> Credits to Quentin Perret <qperret@...gle.com> for the idea of moving
> private VA allocation out of __pkvm_create_private_mapping()
> 
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> ---
> 
> Changes in v4:
>   - Handle null ptr in pkvm_alloc_private_va_range() and replace
>     IS_ERR_OR_NULL checks in callers with IS_ERR checks, per Fuad
>   - Fix kernel-doc comments format, per Fuad
>   - Format __pkvm_create_private_mapping() prototype args (< 80 col), per Fuad
> 
> Changes in v3:
>   - Handle null ptr in IS_ERR_OR_NULL checks, per Mark
> 
> Changes in v2:
>   - Allow specifying an alignment for the private VA allocations, per Marc

I probably badly expressed my earlier concern.

Yes, an alignment is necessary. But how often do we want an alignment
that isn't naturally aligned to the size of the allocation (i.e. the
power of 2 >= the size of the allocation)? This is what the rest of
the kernel does (get_order() and co), and I thing we should follow
this.

This applies to both this patch and the previous one.

Thanks,

	M.

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