linux-kernel - Re: [PATCH v6 4/5] x86: perf: Refactor misc flag assignments

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Message-ID: <597dbcf6-8169-4084-881c-8942ed363189@linux.intel.com>
Date: Wed, 6 Nov 2024 15:33:30 -0500
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: Colton Lewis <coltonlewis@...gle.com>, kvm@...r.kernel.org,
 Sean Christopherson <seanjc@...gle.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Will Deacon <will@...nel.org>,
 Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Naveen N Rao <naveen@...nel.org>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v6 4/5] x86: perf: Refactor misc flag assignments

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.
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.

Thanks,
Kan