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X-CSE-ConnectionGUID: WPP1UHfxR1qcCs4LVrtL3w== X-CSE-MsgGUID: 2bQ3nadrTE6IbkxBEW/x1Q== X-IronPort-AV: E=McAfee;i="6800,10657,11463"; a="74580772" X-IronPort-AV: E=Sophos;i="6.16,234,1744095600"; d="scan'208";a="74580772" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by orvoesa101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Jun 2025 08:19:21 -0700 X-CSE-ConnectionGUID: YGbkUkWxQWOn34IN0SjxJw== X-CSE-MsgGUID: ApgAb0oMQLK5l6VHYpj8Og== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,234,1744095600"; d="scan'208";a="178747752" Received: from spandruv-desk1.amr.corp.intel.com (HELO [10.125.111.228]) ([10.125.111.228]) by orviesa002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Jun 2025 08:19:21 -0700 Message-ID: Date: Fri, 13 Jun 2025 08:19:20 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 03/12] x86/fpu/xstate: Add xsaves_nmi To: "Liang, Kan" , peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, tglx@linutronix.de, dave.hansen@linux.intel.com, irogers@google.com, adrian.hunter@intel.com, jolsa@kernel.org, alexander.shishkin@linux.intel.com, linux-kernel@vger.kernel.org Cc: dapeng1.mi@linux.intel.com, ak@linux.intel.com, zide.chen@intel.com References: <20250613134943.3186517-1-kan.liang@linux.intel.com> <20250613134943.3186517-4-kan.liang@linux.intel.com> <03cfdc36-a7b9-4098-bc21-cb612907add2@linux.intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/13/25 07:54, Liang, Kan wrote: >> One other thing... >> >> XSAVES uses the modified optimization. That means if you did something >> like this: >> >> NMI=> >> xsaves_nmi(); >> <=IRET >> ... run a little bit in the kernel >> NMI=> // another NMI >> xsaves_nmi(); >> <=IRET >> >> The second XSAVES might not actually write anything to the buffer >> because the registers didn't change (they weren't modified). Is that OK? > Yes. The per-cpu buffer in perf is only used by this XSAVES. No one will > clear it or modify it between the two xsaves_nmi(). What prevents it from being freed, reallocated to some other user, freed and reallocated to the per-cpu buffer? ;) But, really, the thing that saves you here is that the modified optimization only comes into effect if you XRSTOR from an XSAVE buffer. This buffer is only ever XSAVES'd to. So I think it's safe ... from the modified optimization. I'll look at the init optimization later in the series.