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From:   Kalesh Singh <kaleshsingh@google.com>
Date:   Thu, 3 Mar 2022 09:45:51 -0800
Message-ID: <CAC_TJvcA8F1x_ynYco9wXeVguOW+OmTB7Z0J-8ES4wVPMxMyFg@mail.gmail.com>
Subject: Re: [PATCH v4 2/8] KVM: arm64: Introduce pkvm_alloc_private_va_range()
To:     Marc Zyngier <maz@kernel.org>
Cc:     Will Deacon <will@kernel.org>, Quentin Perret <qperret@google.com>,
        Fuad Tabba <tabba@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        "Cc: Android Kernel" <kernel-team@android.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
        Andrew Scull <ascull@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@lists.infradead.org>,
        kvmarm <kvmarm@lists.cs.columbia.edu>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Mar 3, 2022 at 9:29 AM Marc Zyngier <maz@kernel.org> wrote:
>
> Hi Kalesh,
>
> On Wed, 02 Mar 2022 17:24:53 +0000,
> Kalesh Singh <kaleshsingh@google.com> wrote:
> >
> > On Tue, Mar 1, 2022 at 11:46 PM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Fri, 25 Feb 2022 03:34:47 +0000,
> > > Kalesh Singh <kaleshsingh@google.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@google.com> for the idea of moving
> > > > private VA allocation out of __pkvm_create_private_mapping()
> > > >
> > > > Signed-off-by: Kalesh Singh <kaleshsingh@google.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?
>
> I don't think we should worry too much about this. Even when building
> the kernel with a very small VA space (commonly 39 bits), we still
> have a quarter of that reserved for private EL2 mappings. That's
> pretty big.
>
> We will use a bit more of the memory that is set aside for EL2 page
> tables, but this shouldn't be a problem either.

Hi Marc,

Thanks for the explanations. I'll update as suggested in the next version.

- Kalesh

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