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X-CSE-ConnectionGUID: KjSrvFYaTsKvqHeFxK4drw== X-CSE-MsgGUID: cLk4mYO9ROC/MbtGN9ZTWw== X-IronPort-AV: E=McAfee;i="6700,10204,11130"; a="17856040" X-IronPort-AV: E=Sophos;i="6.09,201,1716274800"; d="scan'208";a="17856040" Received: from fmviesa008.fm.intel.com ([10.60.135.148]) by fmvoesa113.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Jul 2024 13:58:34 -0700 X-CSE-ConnectionGUID: otJWUUxcTO6T7RZeNMngxA== X-CSE-MsgGUID: gvmJQ3vbTYCNWgp69xYBow== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.09,201,1716274800"; d="scan'208";a="48665160" Received: from unknown (HELO [10.124.221.144]) ([10.124.221.144]) by fmviesa008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Jul 2024 13:58:32 -0700 Message-ID: <1c2fd06e-2e97-4724-80ab-8695aa4334e7@intel.com> Date: Thu, 11 Jul 2024 13:58:30 -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: [PATCH 0/6] Introduce CET supervisor state support To: "Yang, Weijiang" , tglx@linutronix.de, x86@kernel.org, seanjc@google.com, pbonzini@redhat.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: peterz@infradead.org, chao.gao@intel.com, rick.p.edgecombe@intel.com, mlevitsk@redhat.com, john.allen@amd.com References: <20240531090331.13713-1-weijiang.yang@intel.com> <67c5a358-0e40-4b2f-b679-33dd0dfe73fb@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 7/8/24 20:17, Yang, Weijiang wrote: > So I'm not sure whether XFEATURE_MASK_KERNEL_DYNAMIC and related changes > are worth or not for this series. > > Could you share your thoughts? First of all, I really do appreciate when folks make the effort to _try_ to draw their own conclusions before asking the maintainers to share theirs. Next time, OK? ;) But here goes. So we've basically got three cases. Here's a fancy table: > https://docs.google.com/spreadsheets/d/e/2PACX-1vROHIgrtHzUJmdlzT7D7tuVzgM8AMlK2XlorvFIJvk-I0NjD7A-T_qntjz7cUJlCScfWGtSfPK30Xtu/pubhtml ... and the same in ASCII Case |IA32_XSS[12] | Space | RFBM[12] | Drop% -----+-------------+-------+----------+------ 1 | 0 | None | 0 | 0.0% 2 | 1 | None | 0 | 0.2% 3 | 1 | 24B? | 1 | 0.2% Case 1 is the baseline of course. Case 2 avoids allocating space for CET and also leans on the kernel to set RFBM[12]==0 and tell the hardware not to write CET-S state. Case 3 wastes the CET-S space in each task and also leans on the hardware init optimization to avoid writing out CET-S space on each XSAVES. #1 is: 0 lines of code. #2 is: 5 files changed, 90 insertions(+), 27 deletions(-) #3 is: very few lines of code, nearing zero #2 and #3 have the same performance. So we're down to choosing between * $BYTES space in 'struct fpu' (on hardware supporting CET-S) or * ~100 loc $BYTES is 24, right? Did I get anything wrong? So, here's my stake in the ground: I think the 100 lines of code is probably worth it. But I also hate complicating the FPU code, so I'm also somewhat drawn to just eating the 24 bytes and moving on. But I'm still in the "case 2" camp. Anybody disagree?