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

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Message-Id: <1636158401.g3t5cp0jke.astroid@bobo.none>
Date:   Sat, 06 Nov 2021 10:29:04 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Juergen Gross <jgross@...e.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct
 kvm

Excerpts from Sean Christopherson's message of November 6, 2021 12:49 am:
> +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?
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/vcpu-xarray
> [2] https://lkml.kernel.org/r/871r65wwk7.wl-maz@kernel.org

Seems like a good idea if it can allow vcpu structs to be allocated to 
preferred nodes.

>> however x86 kvm_svm uses its own scheme rather than kvm_arch for some reason.
> 
> What's the problem in kvm_svm?

It embeds a struct kvm so it couldn't be variable sized.

Thanks,
Nick