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

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Date:	Wed, 30 Mar 2016 11:06:21 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Julien Grall <julien.grall@....com>,
	kvmarm@...ts.cs.columbia.edu, fu.wei@...aro.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, wei@...hat.com,
	al.stone@...aro.org, gg@...mlogic.co.uk,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 1/9] clocksource: arm_arch_timer: Gather KVM specific
 information in a structure

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@....com>
> > 
> >> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >> Cc: Marc Zyngier <marc.zyngier@....com>
> >>
> >>      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?

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.

Thanks,
-Christoffer