Re: [Qemu-devel] [PATCH v4 4/6] vhost: add regions to temporary list

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/12/2017 19:08, Dr. David Alan Gilbert (git) wrote:
> +    if (dev->tmp_mem->nregions) {
> +        /* Since we already have at least one region, lets see if
> +         * this extends it; since we're scanning in order, we only
> +         * have to look at the last one, and the FlatView that calls
> +         * us shouldn't have overlaps.
> +         */
> +        struct vhost_memory_region *prev_vmr = dev->tmp_mem->regions +
> +                                               (dev->tmp_mem->nregions - 1);
> +        uint64_t prev_gpa_start = prev_vmr->guest_phys_addr;
> +        uint64_t prev_gpa_end   = range_get_last(prev_gpa_start,
> +                                                 prev_vmr->memory_size);
> +        uint64_t prev_host_start = prev_vmr->userspace_addr;
> +        uint64_t prev_host_end   = range_get_last(prev_host_start,
> +                                                  prev_vmr->memory_size);
> +
> +        if (prev_gpa_end + 1 == mrs_gpa &&
> +            prev_host_end + 1 == mrs_host &&
> +            (!dev->vhost_ops->vhost_backend_can_merge ||
> +                dev->vhost_ops->vhost_backend_can_merge(dev,
> +                    mrs_host, mrs_size,
> +                    prev_host_start, prev_vmr->memory_size))) {
> +            /* The two regions abut */
> +            need_add = false;
> +            mrs_size = mrs_size + prev_vmr->memory_size;
> +            prev_vmr->memory_size = mrs_size;
> +            trace_vhost_region_add_tmp_abut(section->mr->name, mrs_size);
> +        }
> +    }

Interesting, in which cases does this actually trigger?

Paolo