Re: [PATCH 3/4] xen/domctl: Introduce fault_ttl

[Jan Beulich <jbeulich@suse.com>]

On 23.12.2020 17:34, Andrew Cooper wrote:
> To inject a simulated resource failure, for testing purposes.
> 
> Given a specific set of hypercall parameters, the failure is in a repeatable
> position, for the currently booted Xen.  The exact position of failures is
> highly dependent on the build of Xen, and hardware support.

What about other kinds of resources, or ones only indirectly
related to memory allocations (e.g. where we don't mean to
associate them with the domain)?

> RFC:
>  * Probably wants to be Kconfig'd

Yes.

>  * I'm thinking of dropping handle from xen_domctl_createdomain because it's a
>    waste of valuable space.

Looks entirely unrelated, but yes - as long as Xen itself has no
consumer of the field. The more that there already is
XEN_DOMCTL_setdomainhandle.

> --- a/xen/common/dmalloc.c
> +++ b/xen/common/dmalloc.c
> @@ -10,7 +10,13 @@ void dfree(struct domain *d, void *ptr)
>  
>  void *_dzalloc(struct domain *d, size_t size, size_t align)
>  {
> -    void *ptr = _xmalloc(size, align);
> +    void *ptr;
> +
> +    if ( atomic_read(&d->fault_ttl) &&
> +         atomic_dec_and_test(&d->fault_ttl) )
> +        return NULL;

Perhaps we want to introduce Linux'es atomic_add_unless()?

Jan