From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 39575F50F; Thu, 12 Feb 2026 09:07:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770887274; cv=none; b=USUufp7/MC0mE7gJ5Nj+jnDtC13KLPK7HkrS3PEibZnpoPS+2rb906iceoPSDKU7dR4+M3UWmHdJ7Rs8f2zFjK+ZCMCGAcg9sQ1254mLZspQsw7rHwNGLEDZyTDaMMt77SqhyIgmr4hTD2PDBYcvcpxADJw55QFlkY8UJcuVy9k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770887274; c=relaxed/simple; bh=7X/ZE+464SFMrcWXG+fw9L92P1t1gtPIejSOE1mm8Sg=; h=Date:Message-ID:From:To:Cc:Subject:In-Reply-To:References: MIME-Version:Content-Type; b=CRYqH6osKWZFuXlKUnqfxev2UCZyTQthLX7UK7YTtsqGd2Tqj2AEMx0ya7YCvyARL+3UHvUXbEUo4zK720XJ6qTwW/vHkf0+cy0jJBArRUnyh/yfVtlWu9NPZT/c9eJdQrqSh15Mk2C2lFdzYDLJO4nvgOmj2pHjoMJwLKInlsM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SRDzJd4y; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SRDzJd4y" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1428BC4CEF7; Thu, 12 Feb 2026 09:07:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770887274; bh=7X/ZE+464SFMrcWXG+fw9L92P1t1gtPIejSOE1mm8Sg=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=SRDzJd4y7pdwghuIRwEX+milR/vWqoeZRao4cczsY/oRSB7rXxGuJLbC8pO7iCSw4 /1fHpKlD9QC8Pr36ty9iBxiTNpXaDNYp9kxfY175hoKc70tInFruadKyuacdByotDw giDuJK9Yv4AbMPDUN5IleiHk09y0vDFg4IFOf1Va0BfXrwqu9s3EcPhdfYle0DOYSg Po0zabmL17OlVZC5nq6r8y/4vDgWU5l1/ykpkYYbjQp+nF7oNrOcToyCcJwcrWAWRT TlUDUSJIbuvgS7XUw4PVNj+qOU1x9wo3jckCVZzmRgQeTQjpQnhNQht/dATAuEMT5X Wv6weRnqIFc7A== Received: from [185.219.108.64] (helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.98.2) (envelope-from ) id 1vqSfv-0000000AYzI-2Zos; Thu, 12 Feb 2026 09:07:51 +0000 Date: Thu, 12 Feb 2026 09:07:33 +0000 Message-ID: <86ldgyba96.wl-maz@kernel.org> From: Marc Zyngier To: Colton Lewis Cc: kvm@vger.kernel.org, Alexandru Elisei , Paolo Bonzini , Jonathan Corbet , Russell King , Catalin Marinas , Will Deacon , Oliver Upton , Mingwei Zhang , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Mark Rutland , Shuah Khan , Ganapatrao Kulkarni , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-perf-users@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v6 09/19] KVM: arm64: Write fast path PMU register handlers In-Reply-To: <20260209221414.2169465-10-coltonlewis@google.com> References: <20260209221414.2169465-1-coltonlewis@google.com> <20260209221414.2169465-10-coltonlewis@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: coltonlewis@google.com, kvm@vger.kernel.org, alexandru.elisei@arm.com, pbonzini@redhat.com, corbet@lwn.net, linux@armlinux.org.uk, catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev, mizhang@google.com, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, mark.rutland@arm.com, shuah@kernel.org, gankulkarni@os.amperecomputing.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-perf-users@vger.kernel.org, linux-kselftest@vger.kernel.org X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Mon, 09 Feb 2026 22:14:04 +0000, Colton Lewis wrote: > > We may want a partitioned PMU but not have FEAT_FGT to untrap the > specific registers that would normally be untrapped. Add a handler for > those registers in the fast path so we can still get a performance > boost from partitioning. > > The idea is to handle traps for all the PMU registers quickly by > writing directly to the hardware when possible instead of hooking into > the emulated vPMU as the standard handlers in sys_regs.c do. This seems extremely premature. My assumption is that PMU traps are rare, and that doing a full exit should be acceptable. Until you demonstrate the contrary, I don't want this sort of massive bloat in the most performance-critical path. "Start walking before you try to run". M. -- Without deviation from the norm, progress is not possible.