Re: [PATCH v9 3/7] KVM: Support dirty ring in conjunction with bitmap

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On Tue, Nov 08, 2022, Gavin Shan wrote:
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 800f9470e36b..228be1145cf3 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -33,6 +33,14 @@ config HAVE_KVM_DIRTY_RING_ACQ_REL
>         bool
>         select HAVE_KVM_DIRTY_RING
>  
> +# Only architectures that need to dirty memory outside of a vCPU
> +# context should select this, advertising to userspace the
> +# requirement to use a dirty bitmap in addition to the vCPU dirty
> +# ring.

The Kconfig does more than advertise a feature to userspace.

 # Allow enabling both the dirty bitmap and dirty ring.  Only architectures that
 # need to dirty memory outside of a vCPU context should select this.

> +config HAVE_KVM_DIRTY_RING_WITH_BITMAP

I think we should replace "HAVE" with "NEED".  Any architecture that supports the
dirty ring can easily support ring+bitmap, but based on the discussion from v5[*],
the comment above, and the fact that the bitmap will _never_ be used in the
proposed implementation because x86 will always have a vCPU, this Kconfig should
only be selected if the bitmap is needed to support migration.

[*] https://lore.kernel.org/all/Y0SxnoT5u7+1TCT+@xxxxxxxxxx

> +	bool
> +	depends on HAVE_KVM_DIRTY_RING
> +
>  config HAVE_KVM_EVENTFD
>         bool
>         select EVENTFD
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index fecbb7d75ad2..f95cbcdd74ff 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -21,6 +21,18 @@ u32 kvm_dirty_ring_get_rsvd_entries(void)
>  	return KVM_DIRTY_RING_RSVD_ENTRIES + kvm_cpu_dirty_log_size();
>  }
>  
> +bool kvm_use_dirty_bitmap(struct kvm *kvm)
> +{
> +	lockdep_assert_held(&kvm->slots_lock);
> +
> +	return !kvm->dirty_ring_size || kvm->dirty_ring_with_bitmap;
> +}
> +
> +bool __weak kvm_arch_allow_write_without_running_vcpu(struct kvm *kvm)

Rather than __weak, what about wrapping this with an #ifdef to effectively force
architectures to implement the override if they need ring+bitmap?  Given that the
bitmap will never be used if there's a running vCPU, selecting the Kconfig without
overriding this utility can't possibly be correct.

  #ifndef CONFIG_NEED_KVM_DIRTY_RING_WITH_BITMAP
  bool kvm_arch_allow_write_without_running_vcpu(struct kvm *kvm)
  {
	return false;
  }
  #endif

> @@ -4588,6 +4608,29 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
>  			return -EINVAL;
>  
>  		return kvm_vm_ioctl_enable_dirty_log_ring(kvm, cap->args[0]);
> +	case KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP: {
> +		int r = -EINVAL;
> +
> +		if (!IS_ENABLED(CONFIG_HAVE_KVM_DIRTY_RING_WITH_BITMAP) ||
> +		    !kvm->dirty_ring_size)

I have no objection to disallowing userspace from disabling the combo, but I
think it's worth requiring cap->args[0] to be '0' just in case we change our minds
in the future.

> +			return r;
> +
> +		mutex_lock(&kvm->slots_lock);
> +
> +		/*
> +		 * For simplicity, allow enabling ring+bitmap if and only if
> +		 * there are no memslots, e.g. to ensure all memslots allocate
> +		 * a bitmap after the capability is enabled.
> +		 */
> +		if (kvm_are_all_memslots_empty(kvm)) {
> +			kvm->dirty_ring_with_bitmap = true;
> +			r = 0;
> +		}
> +
> +		mutex_unlock(&kvm->slots_lock);
> +
> +		return r;
> +	}
>  	default:
>  		return kvm_vm_ioctl_enable_cap(kvm, cap);
>  	}
> -- 
> 2.23.0
> 



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