Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

[Tim Chen]

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.

Thanks.

Tim

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