linux-kernel - Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

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Message-ID: <0ce1757e3267df037912b303f60c662c45d0a6e6.camel@amazon.co.uk>
Date: Mon, 1 Dec 2025 14:45:01 +0000
From: "Woodhouse, David" <dwmw@...zon.co.uk>
To: "Sieber, Fernand" <sieberf@...zon.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"peterz@...radead.org" <peterz@...radead.org>
CC: "Saenz Julienne, Nicolas" <nsaenz@...zon.es>, "Busse, Anselm"
	<abusse@...zon.de>, "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de"
	<bp@...en8.de>, "stable@...r.kernel.org" <stable@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Schönherr, Jan H. <jschoenh@...zon.de>, "Borghorst,
 Hendrik" <hborghor@...zon.de>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "nh-open-source@...zon.com"
	<nh-open-source@...zon.com>
Subject: Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

On Mon, 2025-12-01 at 16:23 +0200, Fernand Sieber wrote
> Perf considers the combination of PERF_COUNT_HW_BRANCH_INSTRUCTIONS with
> a sample_period of 1 a special case and handles this as a BTS event (see
> intel_pmu_has_bts_period()) -- a deviation from the usual semantic,
> where the sample_period represents the amount of branch instructions to
> encounter before the overflow handler is invoked.

That's kind of awful, and seems to be the real underlying cause of the KVM
issue. Can we kill it with fire? Peter?

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