Path: csiph.com!aioe.org!news.servidellagleba.it!bofh.it!news.nic.it!robomod From: Yu-cheng Yu Newsgroups: linux.kernel Subject: Re: [PATCH v2] x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state Date: Fri, 10 Feb 2017 19:30:02 +0100 Message-ID: References: X-Original-To: Rik van Riel X-Extloop1: 1 X-Ironport-Av: E=Sophos;i="5.35,142,1484035200"; d="scan'208";a="57330976" MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Mutt/1.5.21 (2010-09-15) Sender: robomod@news.nic.it List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Approved: robomod@news.nic.it Lines: 41 Organization: linux.* mail to news gateway X-Original-Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, luto@kernel.org, dave.hansen@linux.intel.com, bp@suse.de, hpa@zytor.com X-Original-Date: Fri, 10 Feb 2017 09:18:21 -0800 X-Original-Message-ID: <20170210171821.GA22519@test-lenovo> X-Original-References: <20170209184347.2ef977b9@annuminas.surriel.com> <20170210004541.GA20800@test-lenovo> <1486688411.2096.31.camel@redhat.com> X-Original-Sender: linux-kernel-owner@vger.kernel.org Xref: csiph.com linux.kernel:1578685 On Thu, Feb 09, 2017 at 08:00:11PM -0500, Rik van Riel wrote: > On Thu, 2017-02-09 at 16:45 -0800, Yu-cheng Yu wrote: > > On Thu, Feb 09, 2017 at 06:43:47PM -0500, Rik van Riel wrote: > > >  /* > > > + * Weird legacy quirk: SSE and YMM states store information in the > > > + * MXCSR and MXCSR_FLAGS fields of the FP area. That means if the > > > FP > > > + * area is marked as unused in the xfeatures header, we need to > > > copy > > > + * MXCSR and MXCSR_FLAGS if either SSE or YMM are in use. > > > + */ > > > +static inline bool xfeatures_need_mxcsr_copy(u64 xfeatures) > > > +{ > > > + if (!(xfeatures & (XFEATURE_MASK_SSE|XFEATURE_MASK_YMM))) > > > + return 0; > > > + > > > + if (xfeatures & XFEATURE_MASK_FP) > > > + return 0; > > > + > > > + return 1; > > > +} > > > > Would you consider separating MXCSR & MXCSR_FLAGS from > > XFEATURE_MASK_FP.  Here we assume if both xstate[0] and xstate[1] > > are being copied, then there is no need to copy MXCSR/MXCSR_FLAGS > > again.  What if only xstate[0] is copied and MXCSR/MXCSR_FLAGS > > is invalid? > > Surely then the CPU would ignore the contents of > MXCSR/MXCSR_FLAGS because the SSE and YMM bits in > the xfeatures header are clear? > > What am I missing? For example in copy_user_to_xstate(), is it possible that only xstate[0] is copied in, but in fact the task already has a valid xstate[1] and MXCSR is overwritten? We can think about this as a separate patch. Thanks, Yu-cheng