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Message-ID: <02fe988e-2b42-9610-6ab5-bd17b0d9fb80@oracle.com>
Date: Fri, 9 Feb 2024 01:00:07 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: Yi Wang <up2wing@...il.com>, x86@...nel.org, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, foxywang@...cent.com
Cc: seanjc@...gle.com, pbonzini@...hat.com, mingo@...hat.com,
tglx@...utronix.de, dave.hansen@...ux.intel.com, hpa@...or.com,
bp@...en8.de, wanpengli@...cent.com, oliver.upton@...ux.dev,
anup@...infault.org, frankja@...ux.ibm.com, imbrenda@...ux.ibm.com,
maz@...nel.org, atishp@...shpatra.org, borntraeger@...ux.ibm.com
Subject: Re: [v3 0/3] KVM: irqchip: synchronize srcu only if needed
Hi Yi,
On 1/24/24 03:34, Yi Wang wrote:
> From: Yi Wang <foxywang@...cent.com>
>
> We found that it may cost more than 20 milliseconds very accidentally
> to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> already.
Would you mind explaining the reason that the *number of VMs* matters, as
KVM_CAP_SPLIT_IRQCHIP is a per-VM cap?
Or it meant it is more likely to have some VM workload impacted by the
synchronize_srcu_expedited() as in prior discussion?
https://lore.kernel.org/kvm/CAN35MuSkQf0XmBZ5ZXGhcpUCGD-kKoyTv9G7ya4QVD1xiqOxLg@mail.gmail.com/
Thank you very much!
Dongli Zhang
>
> The reason is that when vmm(qemu/CloudHypervisor) invokes
> KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> might_sleep and kworker of srcu may cost some delay during this period.
> One way makes sence is setup empty irq routing when creating vm and
> so that x86/s390 don't need to setup empty/dummy irq routing.
>
> Note: I have no s390 machine so the s390 patch has not been tested.
>
> Changelog:
> ----------
> v3:
> - squash setup empty routing function and use of that into one commit
> - drop the comment in s390 part
>
> v2:
> - setup empty irq routing in kvm_create_vm
> - don't setup irq routing in x86 KVM_CAP_SPLIT_IRQCHIP
> - don't setup irq routing in s390 KVM_CREATE_IRQCHIP
>
> v1: https://urldefense.com/v3/__https://lore.kernel.org/kvm/20240112091128.3868059-1-foxywang@tencent.com/__;!!ACWV5N9M2RV99hQ!LjwKfBaGVl3u1l9YQSskg_1RU6278h2-fYnYLsoihF9i43aq73eIDqolGzOmeRvO8UlPreQHLqXEL1bAuw$
>
> Yi Wang (3):
> KVM: setup empty irq routing when create vm
> KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP
> KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP
>
> arch/s390/kvm/kvm-s390.c | 9 +--------
> arch/x86/kvm/irq.h | 1 -
> arch/x86/kvm/irq_comm.c | 5 -----
> arch/x86/kvm/x86.c | 3 ---
> include/linux/kvm_host.h | 1 +
> virt/kvm/irqchip.c | 19 +++++++++++++++++++
> virt/kvm/kvm_main.c | 4 ++++
> 7 files changed, 25 insertions(+), 17 deletions(-)
>
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