linux-kernel - Re: [PATCH v5 1/8] KVM: arm64: Introduce hyp_alloc_private_va

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Message-ID: <CAC_TJvc9o5OJ-aSLkDOu8h3jnuHvBQZnh8pRFbHxunVWUyhHzA@mail.gmail.com>
Date:   Wed, 9 Mar 2022 09:04:01 -0800
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Stephen Boyd <swboyd@...omium.org>, Will Deacon <will@...nel.org>,
        Marc Zyngier <maz@...nel.org>, 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>,
        "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 v5 1/8] KVM: arm64: Introduce hyp_alloc_private_va_range()

On Wed, Mar 9, 2022 at 8:50 AM Quentin Perret <qperret@...gle.com> wrote:
>
> On Tuesday 08 Mar 2022 at 15:09:18 (-0800), Kalesh Singh wrote:
> > On Tue, Mar 8, 2022 at 12:21 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > > It looks odd to use an error pointer casted to unsigned long to return
> > > from an address allocation function. Why not pass a pointer for base
> > > like the function was written before and return an int from this
> > > function with 0 for success and negative error value?Otherwise some
> > > sort of define should made like DMA_MAPPING_ERROR and that can be used
> > > to indicate to the caller that the allocation failed, or a simple zero
> > > may work?
> >
> > I wanted to keep consistent between the pkvm and traditional nvhe
> > code. I will refactor both *alloc_private_va_range() functions to take
> > a pointer and return an int error if that's preferred. There would
> > still be a case of this kind of cast in
> > __pkvm_create_private_mapping() which does return an unsigned long
> > address or ERR_PTR(...). It looks like it was made to return the
> > address to facilitate use as a hypercall (@Quentin CMIW).
>
> Yep, passing everything by value was much easier to cross the EL1/EL2
> boundary as that avoids having the hypervisor map kernel memory and all
> that fun. But Stephen's point is fair, so no objection from to keep this
> little dance confined to the hypercall wrapper and make the function
> signature nicer and easier to use for the rest of the code.

Thanks for clarifying Quentin. That sounds good to me.

- Kalesh

>
> Cheers,
> Quentin