Re: [PATCH v3 2/2] vhost: Only align sections for vhost-user

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/01/20 21:24, Dr. David Alan Gilbert (git) wrote:
> +    if (dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER) {   
> +        /* Round the section to it's page size */
> +        /* First align the start down to a page boundary */
> +        size_t mrs_page = qemu_ram_pagesize(mrs_rb);
> +        uint64_t alignage = mrs_host & (mrs_page - 1);
> +        if (alignage) {
> +            mrs_host -= alignage;
> +            mrs_size += alignage;
> +            mrs_gpa  -= alignage;
> +        }
> +        /* Now align the size up to a page boundary */
> +        alignage = mrs_size & (mrs_page - 1);
> +        if (alignage) {
> +            mrs_size += mrs_page - alignage;
> +        }
> +        trace_vhost_region_add_section_aligned(section->mr->name, mrs_gpa, mrs_size,
> +                                               mrs_host);
> +    }

Ok, now I understand!

So it seems to me that the vhost-user protocol is deficient, it should
have had two different fields for the region size and the region
alignment (so that mmap does not fail).  But I guess that's just yet
another thing to remember for vhost-user v2.

I would add a comment to explain why the alignment is needed in the
first place, but this fix is certainly much more pleasant.  Thanks very
much.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo