From nobody Sun Oct 5 09:07:15 2025 Received: from mail-pl1-f201.google.com (mail-pl1-f201.google.com [209.85.214.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6346C2E2EEE for ; Wed, 6 Aug 2025 19:58:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754510332; cv=none; b=UESrxTDbk58JRCUq8pkxj3ZKwlteVWBpMn8wbg1+ZVRDFOFVxd5Wa4MfgL/Cu8RB7XTfnt4puLjTHaTMRUKJhm3OhAWxGIMsHTMhJcWeB1DGtIClRL0n5yN/NWwHaHIAy7ufT2EoR1cL2fHxRzxMEIA9mXSou1dNDtaa7hNBuJc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754510332; c=relaxed/simple; bh=UqKbQ/MEitFcIh6oJaDa5iQaYpCTlGUBLK908ME1g4Q=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=oswrb/dj373n0EgM9K2Ay0fk7Ua/zqFUzqt+4oPM1FpNNI9ri6IM2dAI6uVY0l+F2g0ivIYQhnByfqNWVTX0p84pzhdgg30VzO46A4zZMtCz0e9nK6KDqb1BQo1adu83ULrwfZa1Dx9k2VA5OKwhQDSgCDUvbCtnyqOtAZA9+/g= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=4DFPLcrp; arc=none smtp.client-ip=209.85.214.201 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="4DFPLcrp" Received: by mail-pl1-f201.google.com with SMTP id d9443c01a7336-24283069a1cso1864885ad.1 for ; Wed, 06 Aug 2025 12:58:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1754510328; x=1755115128; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=FEfQdODxvc/WYDgKcBJNCqpRp5QcBVEYBtlnYDr8f+A=; b=4DFPLcrpXC8nzPtT2ABri2kiMNcEVcSAxV5rGGrD+ycZK7+G/xw3XhsrSAq1GUn+KV c3mpEz7/DDiuZDG7GsCIPbif7RO/cx41LAnNr0vTSFC6UyiRf85KJwYiHHN+6gMvD2x3 r/KZj9P6pt0zyfbT2R5csi8Lws+uVx2QP1ChHeoVC3qnHlqdtPqLRPMYbMDeSoA+untO RvTmh5AQsiEoV3GWBjr95m3BQ/5EUFHIiRvNkBOZmvg7CjaLz86K424skxFJa2S2WDzF 5659bUKO7cI22AYTqU4s+3QVJ2DpO+fanRhX7RIBIsPgxSskBdF4PvKFLbevBywWkkOk jgeg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1754510328; x=1755115128; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=FEfQdODxvc/WYDgKcBJNCqpRp5QcBVEYBtlnYDr8f+A=; b=KSFQ6o0aAemBS7drydeJH3jP7w+nwiDPuxNTBTXzMuAiuBBfDqDsArk1U2n+GWcq9X 1fIjfGrUAkqIS3X6vnEhvZLfYd+dUJkUjxk07/foCaKfO7xGh4NU31HNLNGYjEwayWW7 3F+t9hru+a+6Zvfzkppb9vxaKItsEVBWsOwW3HICgttZpZRarzlcP9a6ZJZYUWh0qDW3 TNZu0w30OnZd2K75tXFsLbWCDlcRmqY5F6In733jAWhiDJLbCULciXL2vUEcrbxYfzb+ NmAwnEtdXEfDXjazFRBAVxYrrghI3iI/hv1jXQemMqQhwVh6UmpJrUYLNa9Wan8lRCFE R3UQ== X-Forwarded-Encrypted: i=1; AJvYcCXCsXZqiBHr6mwP5DZK2IiAZUZkBus6SLuoj8kD19gZ9Jrjyyj9tw4XVLMI/nhPg7swXqJkndxu/ZBVhNw=@vger.kernel.org X-Gm-Message-State: AOJu0Yz85KYIVJU/HLkSv1/1bArVL7jgLHDHb9uf4gEADhxxupAldy+S /Zi1acrG+jsBBk+JJIkkSKjmATIcKBKD/eSN3zXagq2iVsDKKdG6hyIgGimaUaDvDl53iH2z6tj 7obBwEA== X-Google-Smtp-Source: AGHT+IHKT98t7sh+NJ88sddK4YezK7lHLHfrTfL2AYLv+RO+SAkhLkiCccC37Go9T+seI3ZwHUra06wFxFw= X-Received: from plbmp11.prod.google.com ([2002:a17:902:fd0b:b0:23f:fa41:1de3]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:3bcc:b0:234:c8ec:51b5 with SMTP id d9443c01a7336-242a0bffb31mr47813405ad.53.1754510327813; Wed, 06 Aug 2025 12:58:47 -0700 (PDT) Reply-To: Sean Christopherson Date: Wed, 6 Aug 2025 12:57:06 -0700 In-Reply-To: <20250806195706.1650976-1-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250806195706.1650976-1-seanjc@google.com> X-Mailer: git-send-email 2.50.1.565.gc32cd1483b-goog Message-ID: <20250806195706.1650976-45-seanjc@google.com> Subject: [PATCH v5 44/44] KVM: x86/pmu: Elide WRMSRs when loading guest PMCs if values already match From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Xin Li , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Sean Christopherson , Paolo Bonzini Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Kan Liang , Yongwei Ma , Mingwei Zhang , Xiong Zhang , Sandipan Das , Dapeng Mi Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" When loading a mediated PMU state, elide the WRMSRs to load PMCs with the guest's value if the value in hardware already matches the guest's value. For the relatively common case where neither the guest nor the host is actively using the PMU, i.e. when all/many counters are '0', eliding the WRMSRs reduces the latency of handling VM-Exit by a measurable amount (WRMSR is significantly more expensive than RDPMC). As measured by KVM-Unit-Tests' CPUID VM-Exit testcase, this provides a a ~25% reduction in latency (4k =3D> 3k cycles) on Intel Emerald Rapids, and a ~13% reduction (6.2k =3D> 5.3k cycles) on AMD Turing. Signed-off-by: Sean Christopherson --- arch/x86/kvm/pmu.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c index ddab1630a978..0e5048ae86fa 100644 --- a/arch/x86/kvm/pmu.c +++ b/arch/x86/kvm/pmu.c @@ -1299,13 +1299,15 @@ static void kvm_pmu_load_guest_pmcs(struct kvm_vcpu= *vcpu) for (i =3D 0; i < pmu->nr_arch_gp_counters; i++) { pmc =3D &pmu->gp_counters[i]; =20 - wrmsrl(gp_counter_msr(i), pmc->counter); + if (pmc->counter !=3D rdpmc(i)) + wrmsrl(gp_counter_msr(i), pmc->counter); wrmsrl(gp_eventsel_msr(i), pmc->eventsel_hw); } for (i =3D 0; i < pmu->nr_arch_fixed_counters; i++) { pmc =3D &pmu->fixed_counters[i]; =20 - wrmsrl(fixed_counter_msr(i), pmc->counter); + if (pmc->counter !=3D rdpmc(INTEL_PMC_FIXED_RDPMC_BASE | i)) + wrmsrl(fixed_counter_msr(i), pmc->counter); } } =20 --=20 2.50.1.565.gc32cd1483b-goog