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Date: Thu, 06 Aug 2015 07:56:10 -0700
From: Dave Hansen <dave@sr71.net>
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To: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Peter Anvin <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>, bp@alien8.de,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, fenghua.yu@intel.com,
        x86@kernel.org, dave.hansen@linux.intel.com
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation
References: <20150728172143.6DDFECA7@viggo.jf.intel.com> <20150805103227.GA3233@gmail.com> <55C21EFC.3060802@sr71.net> <20150806071545.GB2194@gmail.com> <CA+55aFxzOj-Ee=DN-_3CMeDeYVsmvmmgoxd3hp4MpRSp+og7AQ@mail.gmail.com> <20150806082746.GA10974@gmail.com>
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I think we have three options.  Here's some rough pseudo-ish-code to 
sketch them out.

/* Option 1, what we have today */
	/* 
	 * This breaks if offset[i]+size[i] != offset[i+1]
	 * or if alignment is in play.  Silly hardware breaks
	 * this today.
	 */
	for (i = 0; i < nr_xstates; i++) {
		if (!enabled_xstate(i))
			continue;
		total_blob_size += xstate_sizes[i];
	}

/* Option 2: search for the end of the last state, probably works, Ingo likes? */
	for (i = 0; i < nr_xstates; i++) {
		if (cpu_has_xsaves && !enabled_xstate(i))
			continue;
		end_of_state = xstate_offsets[i] + xstate_sizes[i];
		if (xstate_is_aligned[i]) /* currently not implemented */
			end_of_state = ALIGN(end_of_state, 64);
		if (end_of_state > total_blob_size)
			total_blob_size = end_of_state;
	}
	/* align unconditionally, maybe??? */
	total_blob_size = ALIGN(total_blob_size, 64);

	/* Double check our obviously bug-free math with what the CPU says */
	if (!cpu_has_xsaves)
		cpuid(0xD0, 0, &check_total_blob_size, ...);
	else
		cpuid(0xD0, 1, &check_total_blob_size, ...);

	WARN_ON(check_total_blob_size != total_blob_size);


/* Option 3, trust the CPU (what Dave's patch does) */
	if (!cpu_has_xsaves)
		cpuid(0xD0, 0, &total_blob_size, ...);
	else
		cpuid(0xD0, 1, &total_blob_size, ...);