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Re: [Xen-devel] [PATCH SpectreV1+L1TF v7 4/9] nospec: introduce evaluate_nospec


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Norbert Manthey <nmanthey@xxxxxxxxx>
  • Date: Mon, 25 Feb 2019 09:18:59 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@xxxxxxxxx>, wipawel@xxxxxxxxx, Julien Grall <julien.grall@xxxxxxx>, David Woodhouse <dwmw@xxxxxxxxxxxx>, "Martin Mazein\(amazein\)" <amazein@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Julian Stecklina <jsteckli@xxxxxxxxx>, Bjoern Doebel <doebel@xxxxxxxxx>
  • Delivery-date: Mon, 25 Feb 2019 08:20:24 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 2/22/19 14:17, Jan Beulich wrote:
>>>> On 21.02.19 at 09:16, <nmanthey@xxxxxxxxx> wrote:
>> --- /dev/null
>> +++ b/xen/include/asm-x86/nospec.h
>> @@ -0,0 +1,38 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/* Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved. 
>> */
>> +
>> +#ifndef _ASM_X86_NOSPEC_H
>> +#define _ASM_X86_NOSPEC_H
>> +
>> +#include <asm/alternative.h>
>> +
>> +/* Allow to insert a read memory barrier into conditionals */
>> +static always_inline bool barrier_nospec_true(void)
>> +{
>> +#ifdef CONFIG_HVM
>> +    alternative("", "lfence", X86_FEATURE_SC_L1TF_VULN);
>> +#endif
>> +    return true;
>> +}
>> +
>> +/* Allow to protect evaluation of conditionasl with respect to speculation 
>> */
>> +#ifdef CONFIG_HVM
>> +#define evaluate_nospec(condition)                                         \
>> +    ((condition) ? barrier_nospec_true() : !barrier_nospec_true())
>> +#else
>> +#define evaluate_nospec(condition) (condition)
>> +#endif
>> +
>> +/* Allow to block speculative execution in generic code */
>> +#define block_speculation() ((void)barrier_nospec_true())
> Seeing Julien's request for switching to inline functions on the Arm side,
> is there anything preventing these two to become inline functions too?

No, there is nothing preventing using functions here. I mostly copied
the macro style from other nospec constructs. I tested the generated
binary for the functions being always_inline, and the result is the same
as when using macros. I will turn the macros into functions that use the
always_inline keyword, as there have been problems with the
barrier_nospec_true function when not using always_inline.

Best,
Norbert




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