Subject: Re: [PATCH v6 4/5] x86: perf: Refactor misc flag assignments

From: Oliver Upton <[email-masked]>
Date: Wed, 6 Nov 2024 12:51:40 -0800

On Wed, Nov 06, 2024 at 03:33:30PM -0500, Liang, Kan wrote:
> On 2024-11-06 3:02 p.m., Oliver Upton wrote:
> > On Wed, Nov 06, 2024 at 11:03:10AM -0500, Liang, Kan wrote:
> >>> +static unsigned long common_misc_flags(struct pt_regs *regs)
> >>> +{
> >>> +	if (regs->flags & PERF_EFLAGS_EXACT)
> >>> +		return PERF_RECORD_MISC_EXACT_IP;
> >>> +
> >>> +	return 0;
> >>> +}
> >>> +
> >>> +unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
> >>> +{
> >>> +	unsigned long guest_state = perf_guest_state();
> >>> +	unsigned long flags = common_misc_flags(regs);
> >>> +
> >>> +	if (guest_state & PERF_GUEST_USER)
> >>> +		flags |= PERF_RECORD_MISC_GUEST_USER;
> >>> +	else if (guest_state & PERF_GUEST_ACTIVE)
> >>> +		flags |= PERF_RECORD_MISC_GUEST_KERNEL;
> >>> +
> >>
> >> The logic of setting the GUEST_KERNEL flag is implicitly changed here.
> >>
> >> For the current code, the GUEST_KERNEL flag is set for !PERF_GUEST_USER,
> >> which include both guest_in_kernel and guest_in_NMI.
> > 
> > Where is the "guest_in_NMI" state coming from? KVM only reports user v.
> > kernel mode.
> 
> I may understand the kvm_arch_pmi_in_guest() wrong.

kvm_arch_pmi_in_guest() is trying to *guess* whether or not an overflow
interrupt caused the most recent VM-exit, implying a counter overflowed
while in the VM. It has no idea what events are loaded on the PMU and
which contexts they're intended to sample in.

It only makes sense to check kvm_arch_pmi_in_guest() if you're dealing with
an event that counts in both host and guest modes and you need to decide who
to sample.

> However, the kvm_guest_state() at least return 3 states.
> 0
> PERF_GUEST_ACTIVE
> PERF_GUEST_ACTIVE | PERF_GUEST_USER
> 
> The existing code indeed assumes two modes. If it's not user mode, it
> must be kernel mode.
> However, the proposed code behave differently, or at least implies there
> are more modes.
> If it's not user mode and sets PERF_GUEST_ACTIVE, it's kernel mode.

A precondition of the call to perf_arch_guest_misc_flags() is that guest
state is nonzero, meaning a vCPU is loaded presently on this CPU.

-- 
Thanks,
Oliver