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SubjectRe: [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@gmail.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
>

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