linux-kernel - Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct kvm

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Message-ID: <87wnlm8n43.wl-maz@kernel.org>
Date:   Fri, 05 Nov 2021 18:37:48 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct kvm

On Fri, 05 Nov 2021 14:49:57 +0000,
Sean Christopherson <seanjc@...gle.com> wrote:
> 
> +Juergen and Marc
> 
> On Fri, Nov 05, 2021, Nicholas Piggin wrote:
> > Increasing the max VCPUs on powerpc makes the kvm_arch member offset
> > great enough that some assembly breaks due to addressing constants
> > overflowing field widths.
> > 
> > Moving the vcpus array to the end of struct kvm prevents this from
> > happening. It has the side benefit that moving the large array out
> > from the middle of the structure should help keep other commonly
> > accessed fields in the same or adjacent cache lines.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> > ---
> > 
> > It would next be possible to now make this a dynamically sized array,
> > and make the KVM_MAX_VCPUS more dynamic
> 
> Marc has a mostly-baked series to use an xarray[1][2] that AFAICT
> would be well received.  That has my vote, assuming it can get into
> 5.16.  Marc or Juergen, are either of you actively working on that?

I've just revived it, as it needed some RISC-V changes. I'll post it
in a jiffy.

	M.

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