From: Sean Christopherson <seanjc@google.com>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Juergen Gross <jgross@suse.com>, Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct kvm
Date: Fri, 5 Nov 2021 14:49:57 +0000 [thread overview]
Message-ID: <YYVElU6u22qxgQIz@google.com> (raw)
In-Reply-To: <20211105034949.1397997-1-npiggin@gmail.com>
+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
>
next prev parent reply other threads:[~2021-11-05 14:50 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-05 3:49 [PATCH] KVM: move struct kvm_vcpu * array to the bottom of struct kvm Nicholas Piggin
2021-11-05 14:49 ` Sean Christopherson [this message]
2021-11-05 18:37 ` Marc Zyngier
2021-11-06 0:29 ` Nicholas Piggin
2021-11-08 15:35 ` Sean Christopherson
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