Re: [Xen-devel] [RFC PATCH] Adding support for coverage informations

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Fri, Feb 01, 2013 at 02:29:04PM +0000, Frediano Ziglio wrote:
> On Thu, 2013-01-31 at 08:51 +0000, Ian Campbell wrote:
> > On Wed, 2013-01-30 at 21:34 +0000, Konrad Rzeszutek Wilk wrote:
> > > > The reason why adding a new hypercall instead of a new sysctl is simply
> > > > because is easier to have a zero cost if you disable coverage
> > > > informations. The best thing would be redirect do_coverage_op to
> > > > do_ni_hypercall using linker options but even two small stub would do
> > > > (these stubs will return ENOSYS instead).
> > > 
> > > I am not sure I follow. Is the sysctl hypercall code path "longer" than
> > > the hypercall path you are introducing? What is the zero cost?
> > 
> > I don't think we care a jot about the performance of this system call
> > when coverage is disabled, it's certainly not a hot path and in any case
> > if it is a NOP it doesn't really matter anyway.
> > 
> > Ian.
> > 
> 
> It's not about the speed, it's about the bytes introduced in Xen binary.

Not sure I follow. What bytes? Just that the code size is bigger b/c it
will go through the sysctl functions? How many bytes of extra code are
talking here? 

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