Re: [PATCH v4 2/4] KVM: arm64: Dirty quota-based throttling of vcpus

[Marc Zyngier <maz@kernel.org>]

On Sat, 21 May 2022 21:29:38 +0100,
Shivam Kumar <shivam.kumar1@nutanix.com> wrote:
> 
> Exit to userspace whenever the dirty quota is exhausted (i.e. dirty count
> equals/exceeds dirty quota) to request more dirty quota.
> 
> Suggested-by: Shaju Abraham <shaju.abraham@nutanix.com>
> Suggested-by: Manish Mishra <manish.mishra@nutanix.com>
> Co-developed-by: Anurag Madnawat <anurag.madnawat@nutanix.com>
> Signed-off-by: Anurag Madnawat <anurag.madnawat@nutanix.com>
> Signed-off-by: Shivam Kumar <shivam.kumar1@nutanix.com>
> ---
>  arch/arm64/kvm/arm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index ecc5958e27fe..5b6a239b83a5 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -848,6 +848,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  	ret = 1;
>  	run->exit_reason = KVM_EXIT_UNKNOWN;
>  	while (ret > 0) {
> +		ret = kvm_vcpu_check_dirty_quota(vcpu);
> +		if (!ret)
> +			break;
>  		/*
>  		 * Check conditions before entering the guest
>  		 */

Why do we need yet another check on the fast path? It seems to me that
this is what requests are for, so I'm definitely not keen on this
approach. I certainly do not want any extra overhead for something
that is only used on migration. If anything, it is the migration path
that should incur the overhead.

	M.

-- 
Without deviation from the norm, progress is not possible.