Subject: Re: [PATCH v6 5/5] perf: Correct perf sampling with guest VMs

From: Oliver Upton <[email-masked]>
Date: Wed, 6 Nov 2024 11:53:07 -0800

On Wed, Nov 06, 2024 at 11:07:53AM -0500, Liang, Kan wrote:
> > +#ifndef perf_arch_guest_misc_flags
> > +static inline unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
> > +{
> > +	unsigned long guest_state = perf_guest_state();
> > +
> > +	if (guest_state & PERF_GUEST_USER)
> > +		return PERF_RECORD_MISC_GUEST_USER;
> > +
> > +	if (guest_state & PERF_GUEST_ACTIVE)
> > +		return PERF_RECORD_MISC_GUEST_KERNEL;
> 
> Is there by any chance to add a PERF_GUEST_KERNEL flag in KVM?

Why do we need another flag? As it stands today, the vCPU is either in
user mode or kernel mode.

> The PERF_GUEST_ACTIVE flag check looks really confusing.

Perhaps instead:

static inline unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
{
	unsigned long guest_state = perf_guest_state();

	if (!(guest_state & PERF_GUEST_ACTIVE))
		return 0;

	return (guest_state & PERF_GUEST_USER) ? PERF_RECORD_MISC_GUEST_USER :
						 PERF_RECORD_MISC_GUEST_KERNEL;
}

-- 
Thanks,
Oliver