linux-kernel - Re: [PATCH] perf/x86: Don't enforce minimum period for KVM guest-only events

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Message-ID: <20231129112019.GG30650@noisy.programming.kicks-ass.net>
Date:   Wed, 29 Nov 2023 12:20:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Like Xu <likexu@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH] perf/x86: Don't enforce minimum period for KVM
 guest-only events

On Tue, Nov 28, 2023 at 05:33:16PM -0800, Sean Christopherson wrote:

> If programming a "period" of 1 puts the host at risk in some way, then I agree
> that this is unsafe and we need a different solution. 

IIRC if you put in -1 on a Nehalem, you end up with an NMI-storm which
wasn't trivial to recover from if at all (it's too long ago and I don't
have ancient hardware like that anymore :/)

> But if the worst case
> scenario is non-determinstic or odd behavior from the guest's perspective, then
> that's the guest's problem (with the caveat that the guest might not have accurate
> Family/Model/Stepping data to make informed decisions).

Things like bdm_limit_period() will cause odd behaviour IIRC, it does
daft things like generate extra PEBS records on overflow and gives
otherwise daft results for PDIR.

glc_limit_period() lacks a useful comment :/