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

[Juergen Gross <jgross@xxxxxxxx>]

On 25/08/17 14:29, Jan Beulich wrote:
>>>> 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?

BTW: the documentation of "transitive" could be better: does specifying
"transitive" permit or disallow the use of transitive grants?


Juergen


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