Re: [V10 PATCH 08/23] PVH xen: Introduce PVH guest type and some basic changes.

[George Dunlap <george.dunlap@eu.citrix.com>]

On 07/08/13 00:26, Mukesh Rathor wrote:
> On Tue, 6 Aug 2013 13:06:37 +0100
> George Dunlap <George.Dunlap@eu.citrix.com> wrote:
>
>> On Tue, Aug 6, 2013 at 12:47 PM, Jan Beulich <JBeulich@suse.com>
>> wrote:
>>>>>> On 06.08.13 at 13:29, George Dunlap
>>>>>> <George.Dunlap@eu.citrix.com> wrote:
>>>> On Wed, Jul 24, 2013 at 2:59 AM, Mukesh Rathor
>>>> <mukesh.rathor@oracle.com> wrote:
>>>>> --- a/xen/arch/x86/x86_64/traps.c
>>>>> +++ b/xen/arch/x86/x86_64/traps.c
>>>>> @@ -141,7 +141,7 @@ void show_registers(struct cpu_user_regs
>>>>> *regs) }
>>>>>   }
>>>>>
>>>>> -void vcpu_show_registers(const struct vcpu *v)
>>>>> +void vcpu_show_registers(struct vcpu *v)
>>>> Rather than doing this (which could potentially mask a bug in which
>>>> something actually *does* get changed),  wouldn't it make more
>>>> sense to make hvm_kernel_mode (and hvm_get_segment_register) be
>>>> const?
>>> That's what I suggested first too, but which turned out not to
>>> work: Down the call tree there is a use of v where a pointer to
>>> non-const is required (iirc in VMX specific code).
>> Then the changelog should say that, preferably the exact function
>> where non-const is needed, so people know why that's necessary without
>> having to do their own looking.
> And the changelog does say it:
>
> "Note, we drop the const qualifier from vcpu_show_registers() to
>   accomodate the hvm function call in guest_kernel_mode()."

I said *exact function*.  guest_kernel_mode() doesn't need it non-const; 
it needs it because of a function that it calls.  That in turn doesn't 
need it non-const either -- it needs it because of the next one down.  
Who  *actually* needs vcpu to be non-const, way down at the bottom?  
That's what I need to know to understand why we can't just change each 
of those functions to const all the way down.

  -George