From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, linux-perf-users@vger.kernel.org,
linux-kernel@vger.kernel.org, Like Xu <likexu@tencent.com>,
Jim Mattson <jmattson@google.com>,
Mingwei Zhang <mizhang@google.com>
Subject: Re: [PATCH] perf/x86: Don't enforce minimum period for KVM guest-only events
Date: Wed, 29 Nov 2023 12:20:19 +0100 [thread overview]
Message-ID: <20231129112019.GG30650@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <ZWaU3HZURCjms5DM@google.com>
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 :/
prev parent reply other threads:[~2023-11-29 11:20 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-07 18:36 [PATCH] perf/x86: Don't enforce minimum period for KVM guest-only events Sean Christopherson
2023-11-07 19:38 ` Mingwei Zhang
2023-11-07 23:02 ` Sean Christopherson
2023-11-07 23:47 ` Mingwei Zhang
2023-11-17 10:32 ` Peter Zijlstra
2023-11-29 1:33 ` Sean Christopherson
2023-11-29 11:20 ` Peter Zijlstra [this message]
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