Re: [PATCH RFC 2/3] xen/domain: add domain hypfs directories

[Julien Grall <julien@xen.org>]

On 10/12/2020 07:54, Jürgen Groß wrote:
> On 09.12.20 17:37, Julien Grall wrote:
>>>   only the syntax used in this document):
>>>   * STRING -- an arbitrary 0-delimited byte string.
>>> @@ -191,6 +192,15 @@ The scheduling granularity of a cpupool.
>>>   Writing a value is allowed only for cpupools with no cpu assigned 
>>> and if the
>>>   architecture is supporting different scheduling granularities.
>>
>> [...]
>>
>>> +
>>> +static int domain_dir_read(const struct hypfs_entry *entry,
>>> +                           XEN_GUEST_HANDLE_PARAM(void) uaddr)
>>> +{
>>> +    int ret = 0;
>>> +    const struct domain *d;
>>> +
>>> +    for_each_domain ( d )
>>
>> This is definitely going to be an issue if you have a lot of domain 
>> running as Xen is not preemptible.
> 
> In general this is correct, but in this case I don't think it will
> be a problem. The execution time for each loop iteration should be
> rather short (in the microsecond range?), so even with 32000 guests
> we would stay way below one second.

The scheduling slice are usually in ms and not second (yet this will 
depend on your scheduler). It would be unacceptable to me if another 
vCPU cannot run for a second because dom0 is trying to list the domain 
via HYPFS.

Cheers,

-- 
Julien Grall