Re: [Xen-devel] [PATCH v4 15/18] xen/mem_sharing: VM forking

[Julien Grall <julien@xen.org>]

Hi Tamas,

On 08/01/2020 17:14, Tamas K Lengyel wrote:
> +static int mem_sharing_fork(struct domain *d, struct domain *cd)
> +{
> +    int rc;
> +
> +    if ( !d->controller_pause_count &&
> +         (rc = domain_pause_by_systemcontroller(d)) )

AFAIU, the parent domain will be paused if it wasn't paused before and 
this will not be unpaused by the same hypercall. Right?

If so, this brings two questions:
     - What would happen if the toolstack decide to unpause the domain?
     - How does the caller knows whether this was paused by the 
hypercall or not?

I would also suggest to document the behavior of the hypercall as this 
is not entirely obvious that the domain will be paused here.

> +        return rc;
> +
> +    cd->max_pages = d->max_pages;
> +    cd->max_vcpus = d->max_vcpus;
> +
> +    /* this is preemptible so it's the first to get done */
> +    if ( (rc = fork_hap_allocation(d, cd)) )
> +        return rc;
> +
> +    if ( (rc = bring_up_vcpus(cd, d->cpupool)) )
> +        return rc;
> +
> +    if ( (rc = hvm_copy_context_and_params(d, cd)) )
> +        return rc;
> +
> +    fork_tsc(d, cd);
> +
> +    cd->parent = d;

How do you ensure that the parent domain will not get destroyed before 
the forked domain? Do you have a refcount somewhere?

> +
> +    return 0;
> +}
> +

Cheers,

-- 
Julien Grall

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