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Date:   Thu, 15 Feb 2018 00:19:04 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, Dave Hansen <dave@sr71.net>,
        hpa@zytor.com, tglx@linutronix.de, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, dwmw@amazon.co.uk,
        linux-tip-commits@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan@infradead.org>
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware
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* Tim Chen <tim.c.chen@linux.intel.com> wrote:

> On 02/14/2018 12:56 AM, Peter Zijlstra wrote:
> 
> > 
> > At the very least this must disable and re-enable preemption, such that
> > we guarantee we inc/dec the same counter. ISTR some firmware calls (EFI)
> > actually are preemptible so that wouldn't work.
> > 
> > Further, consider:
> > 
> > 	this_cpu_inc_return()		// 0->1
> > 	<NMI>
> > 		this_cpu_inc_return()	// 1->2
> > 		call_broken_arse_firmware()
> > 		this_cpu_dec_return()	// 2->1
> > 	</NMI>
> > 	wrmsr(SPEC_CTRL, IBRS);
> > 
> > 	/* from dodgy firmware crap */
> > 
> > 	this_cpu_dec_return()		// 1->0
> > 	wrmsr(SPEC_CTRL, 0);
> > 
> 
> How about the following patch.

These fragile complications of the interface should now be unnecessary, as the 
only driver that called firmware from NMI callbacks (hpwdt.c) is going to remove 
those firmware callbacks in the near future - solving the problem at the source.

Thanks,

	Ingo