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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.08.17 at 14:05, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 25/08/17 10:49, Jan Beulich wrote:
>>>>> On 24.08.17 at 16:50, <andrew.cooper3@xxxxxxxxxx> 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.
>> Btw, with the need to use v2 on huge systems I'm no longer
>> convinced providing an option to disable it is a good idea.
> 
> "necessary to support larger systems" is not a valid reason to prevent
> smaller systems having the option to reduce their hypervisor attack surface.

Well, yes, one can view it that way. Two questions then, though:
1) If at some point someone comes up with a much better quality
v3, how will your option syntax fit that (i.e. to exclude just v2)?
2) Switching between versions (post-migration) requires extra code
in guests, albeit perhaps not very much. Is it wise to require OSes
to be capable of switching back and forth?

Jan


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