linux-kernel - Re: [PATCH 1/2] KVM: arm64: Introduce KVM_PGTABLE_S2

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Date:   Wed, 17 Mar 2021 14:47:25 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     catalin.marinas@....com, will@...nel.org, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        android-kvm@...gle.com, seanjc@...gle.com,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, tabba@...gle.com, ardb@...nel.org,
        mark.rutland@....com, dbrazdil@...gle.com, mate.toth-pal@....com
Subject: Re: [PATCH 1/2] KVM: arm64: Introduce KVM_PGTABLE_S2_NOFWB Stage-2
 flag

On Wednesday 17 Mar 2021 at 14:41:31 (+0000), Marc Zyngier wrote:
> Hi Quentin,
> 
> On Wed, 17 Mar 2021 14:17:13 +0000,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > In order to further configure stage-2 page-tables, pass flags to the
> > init function using a new enum.
> > 
> > The first of these flags allows to disable FWB even if the hardware
> > supports it as we will need to do so for the host stage-2.
> > 
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > 
> > ---
> > 
> > One question is, do we want to use stage2_has_fwb() everywhere, including
> > guest-specific paths (e.g. kvm_arch_prepare_memory_region(), ...) ?
> > 
> > That'd make this patch more intrusive, but would make the whole codebase
> > work with FWB enabled on a guest by guest basis. I don't see us use that
> > anytime soon (other than maybe debug of some sort?) but it'd be good to
> > have an agreement.
> 
> I'm not sure how useful that would be. We fought long and hard to get
> FWB, and I can't see a good reason to disable it for guests unless the
> HW was buggy (but in which case that'd be for everyone). I'd rather
> keep the changes small for now (this whole series is invasive
> enough!).

OK, that works for me.

> As for this patch, I only have a few cosmetic comments:

Happy with the suggestions, I'll fold that in v6.

Cheers,
Quentin