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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/08/13 00:26, Mukesh Rathor wrote:
> On Tue, 6 Aug 2013 13:06:37 +0100
> George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
>
> > On Tue, Aug 6, 2013 at 12:47 PM, Jan Beulich <JBeulich@xxxxxxxx>
> > wrote:
> > > > > > On 06.08.13 at 13:29, George Dunlap
> > > > > > <George.Dunlap@xxxxxxxxxxxxx> wrote:
> > > > On Wed, Jul 24, 2013 at 2:59 AM, Mukesh Rathor
> > > > <mukesh.rathor@xxxxxxxxxx> 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

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