RE: [PATCH] xen/arm: avoid vtimer flip-flop transition in context switch

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 2022年6月15日 17:47
> To: Wei Chen <Wei.Chen@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: nd <nd@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand
> Marquis <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
> <Volodymyr_Babchuk@xxxxxxxx>
> Subject: Re: [PATCH] xen/arm: avoid vtimer flip-flop transition in context
> switch
> 
> Hi Wei,
> 
> Title: I don't quite understand what you mean by "flip-flop transition".
> 

Sorry for the no accurate words. I mean the time reaches to the MAX uint64_t
and continue from 0. Maybe an "overflow" be better for this description. 

> On 15/06/2022 02:39, Wei Chen wrote:
> > virt_vtimer_save is calculating the new time for the vtimer and
> > has a potential risk of timer flip in:
> > "v->arch.virt_timer.cval + v->domain->arch.virt_timer_base.offset
> > - boot_count".
> > In this formula, "cval + offset" could make uint64_t overflow.
> > Generally speaking, this is difficult to trigger.
> > But unfortunately
> > the problem was encountered with a platform where the timer started
> > with a very huge initial value, like 0xF333899122223333. On this
> > platform cval + offset is overflowing after running for a while.
> 
> I am not sure how this is a problem. Yes, uint64_t will overflow with
> "cval + offset", but AFAIK the overall result will still be correct and
> not undefined.
> 

Yes, just as you said, even overflow, but the result is still correct.
I had just noticed the overflow, but thought in a wrong way.

We have encountered this overflow in one RTOS guest:
PCNT:      0xf33ad45367e4a4ff
EL2_OFF:   0xf333333359e29a7f
BOOT_TICK: 0xf333333359e00000
VCNT:      0x0007a1200e020a80
If there is no timer in list, RTOS will calculate a huge number for
"infinite wait", for example:
VCAL:    0x0ff7a1200e020a80
EL2_OFF + VCAL - BOOT_TICK = 0x032ad45367e4a4ff - 0xf333333359e00000 = 
0xFF7A1200E04A4FF
EL2_OFF - BOOT_TICK + VCAL = 0x29a7f + 0x0ff7a1200e020a80 = 0xFF7A1200E04A4FF


> >
> > So in this patch, we adjust the formula to use "offset - boot_count"
> > first, and then use the result to plus cval. This will avoid the
> > uint64_t overflow.
> 
> Technically, the overflow is still present because the (offset -
> boot_count) is a non-zero value *and* cval is a 64-bit value.
> 

Yes, GuestOS can issue any valid 64-bit value for their usage.

> So I think the equation below should be reworked to...
> 
> >
> > Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> > ---
> >   xen/arch/arm/vtimer.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
> > index 5bb5970f58..86e63303c8 100644
> > --- a/xen/arch/arm/vtimer.c
> > +++ b/xen/arch/arm/vtimer.c
> > @@ -144,8 +144,9 @@ void virt_timer_save(struct vcpu *v)
> >       if ( (v->arch.virt_timer.ctl & CNTx_CTL_ENABLE) &&
> >            !(v->arch.virt_timer.ctl & CNTx_CTL_MASK))
> >       {
> > -        set_timer(&v->arch.virt_timer.timer, ticks_to_ns(v-
> >arch.virt_timer.cval +
> > -                  v->domain->arch.virt_timer_base.offset - boot_count));
> > +        set_timer(&v->arch.virt_timer.timer,
> > +                  ticks_to_ns(v->domain->arch.virt_timer_base.offset -
> > +                              boot_count + v->arch.virt_timer.cval));
> 
> ... something like:
> 
> ticks_to_ns(offset - boot_count) + ticks_to_ns(cval);
> 
> The first part of the equation should always be the same. So it could be
> stored in struct domain.
> 

If you think there is still some values to continue this patch, I will
address this comment : )


Thanks,
Wei Chen

> Cheers,
> 
> --
> Julien Grall