linux-kernel - Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear

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Date:   Thu, 23 Feb 2023 09:03:08 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
        Michael Larabel <michael@...haellarabel.com>,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-mm@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear_young()

On Thu, 23 Feb 2023 03:58:47 +0000,
Yu Zhao <yuzhao@...gle.com> wrote:
> 
> On Fri, Feb 17, 2023 at 2:00 AM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On Fri, 17 Feb 2023 04:21:28 +0000,
> > Yu Zhao <yuzhao@...gle.com> wrote:
> > >
> > > On Thu, Feb 16, 2023 at 9:12 PM Yu Zhao <yuzhao@...gle.com> wrote:
> > > >
> > > > This patch adds kvm_arch_test_clear_young() for the vast majority of
> > > > VMs that are not pKVM and run on hardware that sets the accessed bit
> > > > in KVM page tables.
> >
> > I'm really interested in how you can back this statement. 90% of the
> > HW I have access to is not FEAT_HWAFDB capable, either because it
> > predates the feature or because the feature is too buggy to be useful.
> 
> This is my expericen too -- most devices are pre v8.2.

And yet you have no issue writing the above. Puzzling.

> 
> > Do you have numbers?
> 
> Let's do a quick market survey by segment. The following only applies
> to ARM CPUs:
> 
> 1. Phones: none of the major Android phone vendors sell phones running
> VMs; no other major Linux phone vendors.

Maybe you should have a reality check and look at what your own
employer is shipping.

> 2. Laptops: only a very limited number of Chromebooks run VMs, namely
> ACRVM. No other major Linux laptop vendors.

Again, your employer disagree.

> 3. Desktops: no major Linux desktop vendors.

My desktop disagree (I send this from my arm64 desktop VM ).

> 4. Embedded/IoT/Router: no major Linux vendors run VMs (Android Auto
> can be a VM guest on QNX host).

This email is brought to you via a router VM on an arm64 box.

> 5. Cloud: this is where the vast majority VMs come from. Among the
> vendors available to the general public, Ampere is the biggest player.
> Here [1] is a list of its customers. The A-bit works well even on its
> EVT products (Neoverse cores).

Just the phone stuff dwarfs the number of cloud hosts.

Hopefully your patches are better than your market analysis...

	M.

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