Re: [PATCH v7 06/14] KVM: Make dirty ring exclusive to dirty bitmap log

[Sean Christopherson]

On Wed, Mar 18, 2020 at 12:37:12PM -0400, Peter Xu wrote:
> There's no good reason to use both the dirty bitmap logging and the
> new dirty ring buffer to track dirty bits.  We should be able to even
> support both of them at the same time, but it could complicate things
> which could actually help little.  Let's simply make it the rule
> before we enable dirty ring on any arch, that we don't allow these two
> interfaces to be used together.
> 
> The big world switch would be KVM_CAP_DIRTY_LOG_RING capability
> enablement.  That's where we'll switch from the default dirty logging
> way to the dirty ring way.  As long as kvm->dirty_ring_size is setup
> correctly, we'll once and for all switch to the dirty ring buffer mode
> for the current virtual machine.
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst |  7 +++++++
>  virt/kvm/kvm_main.c            | 12 ++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 99ee9cfc20c4..8f3a83298d3f 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6202,3 +6202,10 @@ make sure all the existing dirty gfns are flushed to the dirty rings.
>  
>  The dirty ring can gets full.  When it happens, the KVM_RUN of the
>  vcpu will return with exit reason KVM_EXIT_DIRTY_LOG_FULL.
> +
> +NOTE: the KVM_CAP_DIRTY_LOG_RING capability and the new ioctl

Leave off "new", it'll be stale a few months/years from now.

> +KVM_RESET_DIRTY_RINGS are exclusive to the existing KVM_GET_DIRTY_LOG

Did you mean "mutually exclusive with"?  "exclusive to" would mean they
can only be used by KVM_GET_DIRTY_LOG with doesn't match the next
sentence.

> +interface.  After enabling KVM_CAP_DIRTY_LOG_RING with an acceptable
> +dirty ring size, the virtual machine will switch to the dirty ring
> +tracking mode, and KVM_GET_DIRTY_LOG, KVM_CLEAR_DIRTY_LOG ioctls will
> +stop working.
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 54a1e893d17b..b289d3bddd5c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1352,6 +1352,10 @@ int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
>  	unsigned long n;
>  	unsigned long any = 0;
>  
> +	/* Dirty ring tracking is exclusive to dirty log tracking */
> +	if (kvm->dirty_ring_size)
> +		return -EINVAL;
> +
>  	*memslot = NULL;
>  	*is_dirty = 0;
>  
> @@ -1413,6 +1417,10 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)
>  	unsigned long *dirty_bitmap_buffer;
>  	bool flush;
>  
> +	/* Dirty ring tracking is exclusive to dirty log tracking */
> +	if (kvm->dirty_ring_size)
> +		return -EINVAL;
> +
>  	as_id = log->slot >> 16;
>  	id = (u16)log->slot;
>  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
> @@ -1521,6 +1529,10 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
>  	unsigned long *dirty_bitmap_buffer;
>  	bool flush;
>  
> +	/* Dirty ring tracking is exclusive to dirty log tracking */
> +	if (kvm->dirty_ring_size)
> +		return -EINVAL;
> +
>  	as_id = log->slot >> 16;
>  	id = (u16)log->slot;
>  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
> -- 
> 2.24.1
>