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

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Message-ID: <CAC_TJvfTZP9+EKFFXQUeGOWJb=WK2Wkd53bPOX2gaz0Gh5o8OA@mail.gmail.com>
Date:   Wed, 2 Mar 2022 09:24:53 -0800
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Will Deacon <will@...nel.org>, Quentin Perret <qperret@...gle.com>,
        Fuad Tabba <tabba@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Cc: Android Kernel" <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>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/8] KVM: arm64: Introduce pkvm_alloc_private_va_range()

On Tue, Mar 1, 2022 at 11:46 PM Marc Zyngier <maz@...nel.org> wrote:
>
> 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.

Hi Marc,

Thanks for clarifying. I think making the alignment implicitly based
on the size here will create unnecessary holes where PAGE_SIZE
alignment would be ok and potentially overflow the private VA space
earlier. Is it not a concern?

- Kalesh
>
> This applies to both this patch and the previous one.
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.
>
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