linux-kernel - Re: [PATCH v6 5/5] perf: Correct perf sampling with guest VMs

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Message-ID: <ZyvJIx-UHXawnUYs@linux.dev>
Date: Wed, 6 Nov 2024 11:53:07 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
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 5/5] perf: Correct perf sampling with guest VMs

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