Re: [PATCH v6 1/9] xen: move do_vcpu_op() to arch specific code

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 27, 2022 at 12:40:41PM +0200, Juergen Gross wrote:
> On 27.06.22 12:28, Roger Pau Monné wrote:
> > On Thu, Mar 24, 2022 at 03:01:31PM +0100, Juergen Gross wrote:
> > > The entry point used for the vcpu_op hypercall on Arm is different
> > > from the one on x86 today, as some of the common sub-ops are not
> > > supported on Arm. The Arm specific handler filters out the not
> > > supported sub-ops and then calls the common handler. This leads to the
> > > weird call hierarchy:
> > > 
> > >    do_arm_vcpu_op()
> > >      do_vcpu_op()
> > >        arch_do_vcpu_op()
> > > 
> > > Clean this up by renaming do_vcpu_op() to common_vcpu_op() and
> > > arch_do_vcpu_op() in each architecture to do_vcpu_op(). This way one
> > > of above calls can be avoided without restricting any potential
> > > future use of common sub-ops for Arm.
> > 
> > Wouldn't it be more natural to have do_vcpu_op() contain the common
> > code (AFAICT handlers for
> > VCPUOP_register_{vcpu_info,runstate_memory_area}) and then everything
> > else handled by the x86 arch_do_vcpu_op() handler?
> > 
> > I find the common prefix misleading, as not all the VCPUOP hypercalls
> > are available to all the architectures.
> 
> This would end up in Arm suddenly supporting the sub-ops it doesn't
> (want to) support today. Otherwise it would make no sense that Arm has
> a dedicated entry for this hypercall.

My preference would be for a common do_vcpu_op() that just contains
handlers for VCPUOP_register_{vcpu_info,runstate_memory_area} and then
an empty arch_ handler for Arm, and everything else moved to the x86
arch_ handler.  That obviously implies some code churn, but results in
a cleaner implementation IMO.

Also has the nice benefit of removing unreachable code from the Arm
build, which is also a MISRA-C rule.

> The "common" just wants to express that the code is common. I'm open
> for a better suggestion, though. :-)

Right, it lives in common/ anyway, so there's not a much better name.

Thanks, Roger.