Re: [Xen-devel] [PATCH] common/gnttab: Introduce command line feature controls

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 08/25/2017 01:31 PM, Jan Beulich wrote:
>>>> On 25.08.17 at 14:10, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 25/08/17 10:57, Jan Beulich wrote:
>>>>>> On 24.08.17 at 17:16, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 24/08/17 16:01, Juergen Gross wrote:
>>>>> On 24/08/17 16:50, Andrew Cooper wrote:
>>>>>> --- a/docs/misc/xen-command-line.markdown
>>>>>> +++ b/docs/misc/xen-command-line.markdown
>>>>>> @@ -868,6 +868,19 @@ Controls EPT related features.
>>>>>>  
>>>>>>  Specify which console gdbstub should use. See **console**.
>>>>>>  
>>>>>> +### gnttab
>>>>>> +> `= List of [ max_ver:<integer>, transitive ]`
>>>>>> +
>>>>>> +> Default: `gnttab=max_ver:2,transitive`
>>>>>> +
>>>>>> +Control various aspects of the grant table behaviour available to 
>>>>>> guests.
>>>>>> +
>>>>>> +* `max_ver` Select the maximum grant table version to offer to guests.  
>>>>>> Valid
>>>>>> +version are 1 and 2.
>>>>>> +* `transitive` Permit or disallow the use of transitive grants.  Note 
>>>>>> that the
>>>>>> +use of grant table v2 without transitive grants is an ABI breakage from 
>>>>>> the
>>>>>> +guests point of view.
>>>>> So shouldn't there be a way for the guest to query the support of
>>>>> transient grants?
>>>> Ideally yes, but how do you suggest doing this in a compatible way?
>>>>
>>>> All Xen downstreams which haven't backported the eventual transitive
>>>> fixes will have this clobber in place, without any query-ability.
>>> That workaround should not be used as an argument to not
>>> provide a way to query the capability. It was put in place knowing
>>> that it would cause problems for (hypothetical) guests using
>>> transitive grants.
>>
>> I am not objecting to introducing a mechanism if a suitable one can be
>> found.
>>
>> However, the heritage of XSA-226 is a valid reason to not block this
>> patch because a mechanism isn't present.
> 
> Code submission deadline for 4.10 isn't very far away; we shouldn't
> ship a major version with a partial workaround.

I'd say we shouldn't ship a major version with a risky, unused feature
on by default.

 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel