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

From: "Liang, Kan" <[email-masked]>
Date: Wed, 6 Nov 2024 15:38:25 -0500

On 2024-11-06 2:53 p.m., Oliver Upton wrote:
> 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;
> }

Yes, this one is much clear.

Can a similar change be done for the x86 perf_arch_guest_misc_flags() in
the previous patch?

Thanks,
Kan