Re: [PATCH v6 1/5] KVM: Implement dirty quota-based throttling of vcpus

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 15.09.22 um 12:10 schrieb Shivam Kumar:
> Define variables to track and throttle memory dirtying for every vcpu.
> 
> dirty_count:    Number of pages the vcpu has dirtied since its creation,
>                  while dirty logging is enabled.
> dirty_quota:    Number of pages the vcpu is allowed to dirty. To dirty
>                  more, it needs to request more quota by exiting to
>                  userspace.
> 
> Implement the flow for throttling based on dirty quota.
> 
> i) Increment dirty_count for the vcpu whenever it dirties a page.
> ii) 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>
[...]

I am wondering if this will work on s390. On s390  we only call
mark_page_dirty_in_slot for the kvm_read/write functions but not
for those done by the guest on fault. We do account those lazily in
kvm_arch_sync_dirty_log (like x96 in the past).

> +
>   void mark_page_dirty_in_slot(struct kvm *kvm,
>   			     const struct kvm_memory_slot *memslot,
>   		 	     gfn_t gfn)
>   {
>   	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
>   
> -#ifdef CONFIG_HAVE_KVM_DIRTY_RING
>   	if (WARN_ON_ONCE(!vcpu) || WARN_ON_ONCE(vcpu->kvm != kvm))
>   		return;
> -#endif

This will rigger on s390 for the interrupt payload written from vhost
if I recall correctly. Please do not enable this warning.

>   
> -	if (memslot && kvm_slot_dirty_track_enabled(memslot)) {
> +	if (!memslot)
> +		return;
> +
> +	WARN_ON_ONCE(!vcpu->stat.generic.pages_dirtied++);
> +
> +	if (kvm_slot_dirty_track_enabled(memslot)) {
>   		unsigned long rel_gfn = gfn - memslot->base_gfn;
>   		u32 slot = (memslot->as_id << 16) | memslot->id;
>   
> @@ -3318,6 +3336,8 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
>   					    slot, rel_gfn);
>   		else
>   			set_bit_le(rel_gfn, memslot->dirty_bitmap);
> +
> +		kvm_vcpu_is_dirty_quota_exhausted(vcpu);
>   	}
>   }
>   EXPORT_SYMBOL_GPL(mark_page_dirty_in_slot);