Re: [PATCH 4/6] Increment dirty counter for vmexit due to page write fault.

[Sean Christopherson <seanjc@google.com>]

On Sun, Nov 14, 2021, Shivam Kumar wrote:
> For a page write fault or "page dirty", the dirty counter of the
> corresponding vCPU is incremented.
> 
> 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>
> Signed-off-by: Shaju Abraham <shaju.abraham@nutanix.com>
> Signed-off-by: Manish Mishra <manish.mishra@nutanix.com>
> ---
>  virt/kvm/kvm_main.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 1564d3a3f608..55bf92cf9f4f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3091,8 +3091,15 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
>  		if (kvm->dirty_ring_size)
>  			kvm_dirty_ring_push(kvm_dirty_ring_get(kvm),
>  					    slot, rel_gfn);
> -		else
> +		else {
> +			struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
> +
> +			if (vcpu && vcpu->kvm->dirty_quota_migration_enabled &&
> +					vcpu->vCPUdqctx)
> +				vcpu->vCPUdqctx->dirty_counter++;

Checking dirty_quota_migration_enabled can race, and it'd be far faster to
unconditionally update a counter, e.g. a per-vCPU stat.

> +
>  			set_bit_le(rel_gfn, memslot->dirty_bitmap);
> +		}
>  	}
>  }
>  EXPORT_SYMBOL_GPL(mark_page_dirty_in_slot);
> -- 
> 2.22.3
>