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

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Date:   Fri, 5 Nov 2021 14:49:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of
 struct kvm

+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

> however x86 kvm_svm uses its own scheme rather than kvm_arch for some reason.

What's the problem in kvm_svm?

> Thanks,
> Nick
> 
>  include/linux/kvm_host.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 0f18df7fe874..78cd9b63a6a5 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -553,7 +553,6 @@ struct kvm {
>  	struct mutex slots_arch_lock;
>  	struct mm_struct *mm; /* userspace tied to this vm */
>  	struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
> -	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
>  
>  	/* Used to wait for completion of MMU notifiers.  */
>  	spinlock_t mn_invalidate_lock;
> @@ -623,6 +622,9 @@ struct kvm {
>  	struct notifier_block pm_notifier;
>  #endif
>  	char stats_id[KVM_STATS_NAME_SIZE];
> +
> +	/* This array can be very large, so keep it at the bottom */
> +	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
>  };
>  
>  #define kvm_err(fmt, ...) \
> -- 
> 2.23.0
>