[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [PATCH] xen/arm: avoid extra caclulations when setting vtimer in context switch
Hi Julien, > -----Original Message----- > From: Julien Grall <julien@xxxxxxx> > Sent: 2022年6月27日 18:36 > To: Jiamei Xie <Jiamei.Xie@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand Marquis > <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk > <Volodymyr_Babchuk@xxxxxxxx>; Wei Chen <Wei.Chen@xxxxxxx> > Subject: Re: [PATCH] xen/arm: avoid extra caclulations when setting vtimer > in context switch > > Hi Jiami > > Title: s/caclulations/calculations/ > > However, I think the title should mention the overflow rather than the > extra calculations. The former is more important the latter. > I will change the title to " xen/arm: avoid overflow when setting vtimer in context switch" > On 27/06/2022 03:58, Jiamei Xie wrote: > > virt_vtimer_save is calculating the new time for the vtimer in: > > "v->arch.virt_timer.cval + v->domain->arch.virt_timer_base.offset > > - boot_count". > > In this formula, "cval + offset" might cause uint64_t overflow. > > Changing it to "v->domain->arch.virt_timer_base.offset - boot_count + > > v->arch.virt_timer.cval" can reduce the possibility of overflow > > This read strange to me. We want to remove the overflow completely not > reducing it. The overflow is completely removed by converting the > "offset - bount_count" to ns upfront. > > AFAICT, the commit message doesn't explain that. Thanks for pointing out that. How about putting the commit message like the below: xen/arm: avoid overflow when setting vtimer in context switch virt_vtimer_save is calculating the new time for the vtimer in: "v->arch.virt_timer.cval + v->domain->arch.virt_timer_base.offset - boot_count". In this formula, "cval + offset" might cause uint64_t overflow. Changing it to "ticks_to_ns(v->domain->arch.virt_timer_base.offset - boot_count) + ticks_to_ns(v->arch.virt_timer.cval)" can avoid overflow, and "ticks_to_ns(arch.virt_timer_base.offset - boot_count)" will be always the same, which has been caculated in domain_vtimer_init. Introduce a new field virt_timer_base.nanoseconds to store this value for arm in struct arch_domain, so we can use it directly. > > > , and > > "arch.virt_timer_base.offset - boot_count" will be always the same, > > which has been caculated in domain_vtimer_init. Introduce a new field > > vtimer_offset.nanoseconds to store this value for arm in struct > > arch_domain, so we can use it directly and extra caclulations can be > > avoided. > > > > This patch is enlightened from [1]. > > > > Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx> > > > > [1] https://www.mail-archive.com/xen- > devel@xxxxxxxxxxxxxxxxxxxx/msg123139.htm > > This link doesn't work. But I would personally remove it from the commit > message (or add ---) because it doesn't bring value (this patch looks > like a v2 to me). Sorry, a 'l' is missing at the end of the link. The link is https://www.mail-archive.com/xen-devel@xxxxxxxxxxxxxxxxxxxx/msg123139.html . I will put it after --- in v3. > > > --- > > xen/arch/arm/include/asm/domain.h | 4 ++++ > > xen/arch/arm/vtimer.c | 6 ++++-- > > 2 files changed, 8 insertions(+), 2 deletions(-) > > > > diff --git a/xen/arch/arm/include/asm/domain.h > b/xen/arch/arm/include/asm/domain.h > > index ed63c2b6f9..94fe5b6444 100644 > > --- a/xen/arch/arm/include/asm/domain.h > > +++ b/xen/arch/arm/include/asm/domain.h > > @@ -73,6 +73,10 @@ struct arch_domain > > uint64_t offset; > > } virt_timer_base; > > > > + struct { > > + int64_t nanoseconds; > > This should be s_time_t to match the argument of set_timer() and return > of ticks_to_ns(). > > > + } vtimer_offset; > > Why are you adding a new structure rather than re-using virt_timer_base? Sure, I'll add this field in virt_timer_base. struct { uint64_t offset; s_time_t nanoseconds; } virt_timer_base; > > > + > > struct vgic_dist vgic; > > > > struct vuart { > > diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c > > index 6b78fea77d..54161e5fea 100644 > > --- a/xen/arch/arm/vtimer.c > > +++ b/xen/arch/arm/vtimer.c > > @@ -64,6 +64,7 @@ int domain_vtimer_init(struct domain *d, struct > xen_arch_domainconfig *config) > > { > > d->arch.virt_timer_base.offset = get_cycles(); > > d->time_offset.seconds = ticks_to_ns(d->arch.virt_timer_base.offset - > boot_count); > > + d->arch.vtimer_offset.nanoseconds = d->time_offset.seconds; > > Hmmm... I find odd to assign a field "nanoseconds" to "seconds". I would > suggest to re-order so you first set nanoseconds and then set seconds. > > This will make more obvious that this is not a mistake and "seconds" > will be closer to the do_div() below. Is it ok to remove do_div and write like below? d->arch.virt_timer_base.nanoseconds = ticks_to_ns(d->arch.virt_timer_base.offset - boot_count); d->time_offset.seconds = d->arch.virt_timer_base.nanoseconds / 1000000000; Best wishes Jiamei Xie > > > do_div(d->time_offset.seconds, 1000000000); > > > > config->clock_frequency = timer_dt_clock_frequency; > > @@ -144,8 +145,9 @@ 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, > > + v->domain->arch.vtimer_offset.nanoseconds + > > + ticks_to_ns(v->arch.virt_timer.cval)); > > } > > } > > > > Cheers, > > -- > Julien Grall
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