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Re: [Xen-devel] [PATCH v2 3/5] xen/domain: Stricter configuration checking


  • To: Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 13 Nov 2018 14:39:43 +0000
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  • Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Tue, 13 Nov 2018 14:39:51 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 13/11/2018 14:36, Wei Liu wrote:
> On Tue, Nov 13, 2018 at 07:14:24AM -0700, Jan Beulich wrote:
>>>>> On 12.11.18 at 17:16, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> Currently, a number of options passed for domain creation are ignored, or 
>>> have
>>> implicit fallback behaviour.  This is bad for forwards compatibility, and 
>>> for
>>> end users to be certain that they got the configuration they asked for.
>>>
>>> With this change:
>>>  * ARM now strictly requires that XEN_DOMCTL_CDF_hap is passed.  Previously,
>>>    only XEN_DOMCTL_CDF_hvm_guest was checked.
>>>  * For x86, requesting HAP without HVM is now prohibited, as the combination
>>>    makes no sense.
>>>  * For x86, requesting HAP on a non-HAP capable system will fail, rather 
>>> than
>>>    silently fall back to Shadow.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>> CC: Julien Grall <julien.grall@xxxxxxx>
>>> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxx>
>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>
>>> Semi RFC because this may cause a user-visible change in behaviour.  
>>> However,
>>> if the user has gone to the effort of specifying hap=1, silently falling 
>>> back
>>> to shadow is unexpected, and IMO, a bug.
>> My view on this to a fair part depends on whether the tool stack
>> would guard us from actually getting into such a situation in the
>> hypervisor. Getting an unspecific -EINVAL back without further
>> help towards diagnosis by the tool stack would make such a
>> change undesirable imo.
> If you want toolstack to tell you what goes wrong, this sanitisation
> function should be shared with the toolstack, and presumably with some
> if __XEN_TOOLS__ trickeries to return / print out the culprit.

Some bits of logic could be shared like that, but some can't.

As a different idea, could we hand back an up-to-128 byte string in the
failure case?  There is space for that in the domctl.u because we've got
no other error information we need to propagate backwards.

~Andrew

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