Re: [PATCH v4 1/9] clocksource: arm_arch_timer: Gather KVM specific information in a structure

[Daniel Lezcano]

On 03/30/2016 11:12 AM, Marc Zyngier wrote:
> On 30/03/16 10:06, Christoffer Dall wrote:
> > On Tue, Mar 29, 2016 at 06:32:15PM +0100, Marc Zyngier wrote:
> > > Daniel,
> > >
> > > On 29/03/16 18:13, Daniel Lezcano wrote:
> > > > On 03/24/2016 06:53 PM, Julien Grall wrote:
> > > > > Introduce a structure which are filled up by the arch timer driver and
> > > > > used by the virtual timer in KVM.
> > > > >
> > > > > The first member of this structure will be the timecounter. More members
> > > > > will be added later.
> > > > >
> > > > > A stub for the new helper isn't introduced because KVM requires the arch
> > > > > timer for both ARM64 and ARM32.
> > > > >
> > > > > The function arch_timer_get_timecounter is kept for the time being and
> > > > > will be dropped in a subsequent patch.
> > > > >
> > > > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > >
> > > > > Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > > > >
> > > > >       Changes in v3:
> > > > >           - Rename the patch
> > > > >           - Move the KVM changes and removal of arch_timer_get_timecounter
> > > > >           in separate patches.
> > > > > ---
> > > > >    drivers/clocksource/arm_arch_timer.c | 12 +++++++++---
> > > > >    include/clocksource/arm_arch_timer.h |  5 +++++
> > > > >    2 files changed, 14 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> > > > > index 5152b38..62bdfe7 100644
> > > > > --- a/drivers/clocksource/arm_arch_timer.c
> > > > > +++ b/drivers/clocksource/arm_arch_timer.c
> > > > > @@ -468,11 +468,16 @@ static struct cyclecounter cyclecounter = {
> > > > >    	.mask	= CLOCKSOURCE_MASK(56),
> > > > >    };
> > > > >
> > > > > -static struct timecounter timecounter;
> > > > > +static struct arch_timer_kvm_info arch_timer_kvm_info;
> > > >
> > > > This structure is statically defined in this subsystem but not used in
> > > > this file and a couple of a accessors is added to let another subsystem
> > > > to access it.
> > > >
> > > > That sounds there is something wrong here with the design of the current
> > > > code, virt/phys are mixed.
> > > >
> > > > It isn't possible to split the virt/phys timer code respectively in
> > > > virt/kvm/arm/arch_timer.c and drivers/clocksource/arm_arch_timer.c ?
> > >
> > > No, that'd be the wrong thing to do. The kernel uses *either* the virt
> > > or phys timer depending on how it has been booted, and both counters are
> > > in use.
> > >
> > > What KVM (or any other hypervisor) needs from the timer subsystem is:
> > > - an interrupt (so that it can force a guest exit when the timer fires),
> > > - a way to convert the values programmed into the HW into a timer event
> > > (which is what the time counter structure is for).
> > >
> > > That allows the hypervisor to *emulate* a timer for the guest, and
> > > that's what virt/kvm/arm/arch_timer.c is all about. We have a clear
> > > separation of what is driving the HW vs what is emulating it, and I'm
> > > quite eager to preserve that.
> > >
> > > > At least, 'struct arch_timer_kvm_info' should belong to
> > > > virt/kvm/arm/arch_timer.c.
> > >
> > > At the cost of mandating separate storage in the arm_arch_timer driver.
> > > I do not find that much nicer, but if you prefer that, fine by me.
> > If arch_timer_kvm_info is declared in virt/kvm/arm/arch_timer.c, then
> > do you want to make it globally accessible and populated by this code or
> > make it static to the KVM code and populate it with accessor functions?
>
> That'd be the latter, as I'm really not fond of global data.
>
> > To me the natural thing is that the arch timer driver maintains data
> > about the device it drives, and consumers of that data can ask the arch
> > timer driver for the details.
>
> That was my approach too, and that's the way the code proposed by Julien
> works. Daniel seems to have a different take on it though.

Well, I'm not against Julien's changes. The arm_arch_timer is complex 
and I don't have all the knowledge for the virt side. So I am just 
asking if everything is clearly separated which seems to be the case 
regarding your previous email.

What sounds strange to me is we have a static global function which is 
not used (except at init time) by the timer and then we add accessors 
function to retrieve it. I would have expected arch_timer to pass a 
structure at init time to the timer driver and this one fills it. Then 
the arch timer can directly use its own structure.

Anyway, perhaps I am splitting hairs. So up to you if you want to keep 
the current approach.

  -- Daniel

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