From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 3D36528D8F1; Thu, 4 Dec 2025 15:17:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764861468; cv=none; b=C4wuGiBjZtjDl3zogS2TLN1mHsjJHIW/wYz1GsRoGjEZ2h3xK2md0/YrHzcfWut5rmnWZ2Jo2e1Wid2CjYux9BhUxCNfPhzSld7/ammoAIuh8uFrBkFIaWnmuKWP7HxhUjE3zavUkTQhDVXxMpFQQyCGHS2TOgz5Mn6el7EF+N0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764861468; c=relaxed/simple; bh=hV57clRW+wpiYylqDXPtkiindbjOMDruNDZBGnNV6J8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SmugHkEVxOjgLtKLuM7HEdJ/yrKlu35HuB9UHJAcaF3wD3HDOmsarV8ntTiZxzZcIVJ30nxVY/6vDPq8CBLJenMSr+x3X7bVgr7P7+qLcb65gsDibtx2WXtwKuUJ7dDy6CZKWqSaA+XZMh2v1TucbM4Si9PVe99xJKn3QWsA67Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=cfDve1FJ; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="cfDve1FJ" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=rvxWzEfFMQOWKLdgZ3xIPHPAEdldbKZZc9FtHIVjgWA=; b=cfDve1FJ/pBF/mMqFr/7Mc9LMA 3dKv6bwmhh/a8zseGq9btrkmFphW/vgVv8ta3fBc7d05WYU0pgob3oqB9Zfjgg0xtQun8AMbtyVOf 2pzRWijooOv4xEFndwx6SJDCv9muWXgfdA9tOxxKMEJq2nnsiAIo6fHYQGcs/z6KoIqTmmV2vPmrb tphVAybUceyzrG33igYV3JDf31pMgPoXBjlkFeZBwHb86RvKCRnVoejeoKciXF+JtWJQBEDViF1XY /AnMPyNZYKIuSGqYKn+8jkAZcKXqf5aoKUus6Z/dE3lOMMINXiBs1mhv6gBMheW0fukdPhRBKeiil MhV5yyNw==; Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vRADp-00000004ICH-23S3; Thu, 04 Dec 2025 14:22:17 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 991083004B8; Thu, 04 Dec 2025 16:17:35 +0100 (CET) Date: Thu, 4 Dec 2025 16:17:35 +0100 From: Peter Zijlstra To: Dapeng Mi Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Thomas Gleixner , Dave Hansen , Ian Rogers , Adrian Hunter , Jiri Olsa , Alexander Shishkin , Andi Kleen , Eranian Stephane , Mark Rutland , broonie@kernel.org, Ravi Bangoria , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Zide Chen , Falcon Thomas , Dapeng Mi , Xudong Hao , Kan Liang Subject: Re: [Patch v5 06/19] perf/x86: Add support for XMM registers in non-PEBS and REGS_USER Message-ID: <20251204151735.GO2528459@noisy.programming.kicks-ass.net> References: <20251203065500.2597594-1-dapeng1.mi@linux.intel.com> <20251203065500.2597594-7-dapeng1.mi@linux.intel.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251203065500.2597594-7-dapeng1.mi@linux.intel.com> On Wed, Dec 03, 2025 at 02:54:47PM +0800, Dapeng Mi wrote: > From: Kan Liang > > While collecting XMM registers in a PEBS record has been supported since > Icelake, non-PEBS events have lacked this feature. By leveraging the > xsaves instruction, it is now possible to snapshot XMM registers for > non-PEBS events, completing the feature set. > > To utilize the xsaves instruction, a 64-byte aligned buffer is required. > A per-CPU ext_regs_buf is added to store SIMD and other registers, with > the buffer size being approximately 2K. The buffer is allocated using > kzalloc_node(), ensuring natural alignment and 64-byte alignment for all > kmalloc() allocations with powers of 2. > > The XMM sampling support is extended for both REGS_USER and REGS_INTR. > For REGS_USER, perf_get_regs_user() returns the registers from > task_pt_regs(current), which is a pt_regs structure. It needs to be > copied to user space secific x86_user_regs structure since kernel may > modify pt_regs structure later. > > For PEBS, XMM registers are retrieved from PEBS records. > > In cases where userspace tasks are trapped within kernel mode (e.g., > during a syscall) when an NMI arrives, pt_regs information can still be > retrieved from task_pt_regs(). However, capturing SIMD and other > xsave-based registers in this scenario is challenging. Therefore, > snapshots for these registers are omitted in such cases. > > The reasons are: > - Profiling a userspace task that requires SIMD/eGPR registers typically > involves NMIs hitting userspace, not kernel mode. > - Although it is possible to retrieve values when the TIF_NEED_FPU_LOAD > flag is set, the complexity introduced to handle this uncommon case in > the critical path is not justified. > - Additionally, checking the TIF_NEED_FPU_LOAD flag alone is insufficient. > Some corner cases, such as an NMI occurring just after the flag switches > but still in kernel mode, cannot be handled. Urgh.. Dave, Thomas, is there any reason we could not set TIF_NEED_FPU_LOAD *after* doing the XSAVE (clearing is already done after restore). That way, when an NMI sees TIF_NEED_FPU_LOAD it knows the task copy is consistent. I'm not at all sure this is complex, it just needs a little care. And then there is the deferred thing, just like unwind, we can defer REGS_USER/STACK_USER much the same, except someone went and built all that deferred stuff with unwind all tangled into it :/