Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

[Peter Zijlstra]

On Fri, Jun 14, 2019 at 12:50:51PM +0100, Dmitry Safonov wrote:
> On 6/14/19 11:08 AM, Vitaly Kuznetsov wrote:
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> > 
> >> @@ -182,7 +182,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
> >>  	struct hv_reenlightenment_control re_ctrl = {
> >>  		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
> >>  		.enabled = 1,
> >> -		.target_vp = hv_vp_index[smp_processor_id()]
> >> +		.target_vp = hv_vp_index[raw_smp_processor_id()]
> >>  	};
> >>  	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
> >>  
> > 
> > Yes, this should do, thanks! I'd also suggest to leave a comment like
> > 	/* 
> >          * This function can get preemted and migrate to a different CPU
> > 	 * but this doesn't matter. We just need to assign
> > 	 * reenlightenment notification to some online CPU. In case this
> >          * CPU goes offline, hv_cpu_die() will re-assign it to some
> >  	 * other online CPU.
> > 	 */
> 
> What if the cpu goes down just before wrmsrl()?
> I mean, hv_cpu_die() will reassign another cpu, but this thread will be
> resumed on some other cpu and will write cpu number which is at that
> moment already down?
> 
> (probably I miss something)
> 
> And I presume it's guaranteed that during hv_cpu_die() no other cpu may
> go down:
> :	new_cpu = cpumask_any_but(cpu_online_mask, cpu);
> :	re_ctrl.target_vp = hv_vp_index[new_cpu];
> :	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));

Then cpus_read_lock() is the right interface, not preempt_disable().

I know you probably can't change the HV interface, but I'm thinking its
rather daft you have to specify a CPU at all for this. The HV can just
pick one and send the notification there, who cares.