Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of domains with a specified or random domid

[Julien Grall <julien@xen.org>]

Hi Paul,

On 13/01/2020 16:54, Durrant, Paul wrote:
>> -----Original Message-----
>> From: jandryuk@gmail.com <jandryuk@gmail.com>
>> Sent: 13 January 2020 16:16
>> To: Durrant, Paul <pdurrant@amazon.co.uk>
>> Cc: xen-devel <xen-devel@lists.xenproject.org>; Anthony PERARD
>> <anthony.perard@citrix.com>; Ian Jackson <ian.jackson@eu.citrix.com>; Wei
>> Liu <wl@xen.org>
>> Subject: Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of domains
>> with a specified or random domid
>>
>> On Thu, Jan 9, 2020 at 6:50 AM Paul Durrant <pdurrant@amazon.com> wrote:
>>>
>>> This patch adds a 'domid' field to libxl_domain_create_info and then
>>> modifies do_domain_create() to use that value if it is valid. Any valid
>>> domid will be checked against the retired domid list before being passed
>>> to libxl__domain_make().
>>> If the domid value is invalid then Xen will choose the domid, as before,
>>> unless the value is the new special RANDOM_DOMID value added to the API.
>>> This value instructs libxl__domain_make() to select a random domid
>> value,
>>> check it for validity, verify it does not match a retired domain, and
>> then
>>> pass it to Xen's XEN_DOMCTL_createdomain operation. If Xen determines
>> that
>>> it co-incides with an existing domain, a new random value will be
>>> selected and the operation will be re-tried.
>>>
>>> NOTE: libxl__logv() is also modified to only log valid domid values in
>>>        messages rather than any domid, valid or otherwise, that is not
>>>        INVALID_DOMID.
>>>
>>> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>>> ---
>>> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
>>> Cc: Wei Liu <wl@xen.org>
>>> Cc: Anthony PERARD <anthony.perard@citrix.com>
>>>
>>> v2:
>>>   - Re-worked to use a value from libxl_domain_create_info
>>> ---
>>>   tools/libxl/libxl.h          |  9 +++++++++
>>>   tools/libxl/libxl_create.c   | 32 +++++++++++++++++++++++++++++++-
>>>   tools/libxl/libxl_internal.c |  2 +-
>>>   tools/libxl/libxl_types.idl  |  1 +
>>>   4 files changed, 42 insertions(+), 2 deletions(-)
>>>
>>
>> <snip>
>>
>>> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
>>> index 1835a5502c..ee76dee364 100644
>>> --- a/tools/libxl/libxl_create.c
>>> +++ b/tools/libxl/libxl_create.c
>>> @@ -600,9 +600,39 @@ int libxl__domain_make(libxl__gc *gc,
>> libxl_domain_config *d_config,
>>>               goto out;
>>>           }
>>>
>>> -        ret = xc_domain_create(ctx->xch, domid, &create);
>>> +        if (libxl_domid_valid_guest(info->domid)) {
>>> +            *domid = info->domid;
>>> +
>>> +            if (libxl__is_retired_domid(gc, *domid)) {
>>> +                LOGED(ERROR, *domid, "domain id is retired");
>>> +                rc = ERROR_FAIL;
>>> +                goto out;
>>> +            }
>>> +        } else if (info->domid == RANDOM_DOMID) {
>>> +            *domid = 0; /* Zero-out initial value */
>>> +        }
>>> +
>>> +        for (;;) {
>>> +            if (info->domid == RANDOM_DOMID) {
>>> +                /* Randomize lower order bytes */
>>> +                ret = libxl__random_bytes(gc, (void *)domid,
>>> +                                          sizeof(uint16_t));
>>
>> Casting to void * assumes little endian.
> 
> I think that's a fairly safe assumption as far as Xen goes...

Not really, there are technically nothing (other than bug fixes) 
preventing us to use a big endian guest on Xen on Arm.

I actually did play with big endian on Xen in the past and managed to 
get a guest running. The main annoying part is Linux as it is assuming 
to use the same endian as the hypervisor. But other OS may not have this 
issues...

The hypervisor itself is likely going to stay little endian, so does the 
interface. For the tools, we should aim to not introduce more assumption 
that the software will be little endian.

Cheers,

-- 
Julien Grall

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