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Re: [Xen-devel] [PATCH SpectreV1+L1TF v4 01/11] is_control_domain: block speculation
- To: Norbert Manthey <nmanthey@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 24 Jan 2019 20:33:50 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@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: Thu, 24 Jan 2019 20:33:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 24/01/2019 12:07, Norbert Manthey wrote:
> On 1/23/19 14:20, Jan Beulich wrote:
>>>>> On 23.01.19 at 12:51, <nmanthey@xxxxxxxxx> wrote:
>>> --- a/xen/include/xen/nospec.h
>>> +++ b/xen/include/xen/nospec.h
>>> @@ -58,6 +58,21 @@ static inline unsigned long
>>> array_index_mask_nospec(unsigned long index,
>>> (typeof(_i)) (_i & _mask); \
>>> })
>>>
>>> +/*
>>> + * allow to insert a read memory barrier into conditionals
>>> + */
>>> +#ifdef CONFIG_X86
>>> +static inline bool lfence_true(void) { rmb(); return true; }
>>> +#else
>>> +static inline bool lfence_true(void) { return true; }
>>> +#endif
>>> +
>>> +/*
>>> + * protect evaluation of conditional with respect to speculation
>>> + */
>>> +#define evaluate_nospec(condition) \
>>> + (((condition) && lfence_true()) || !lfence_true())
>> It may be just me, but I think
>>
>> #define evaluate_nospec(condition) \
>> ((condition) ? lfence_true() : !lfence_true())
>>
>> would better express the two-way nature of this.
> I compared the binary output of the two variants, and they are the same
> (for my build environment). I'll switch to your variant, in case nobody
> objects.
Is it safe though? The original variant is required by C to only
evaluate one of the lfence_true() blocks, whereas the second variation
could execute both of them and cmov the 1 and 0 together, which is wasteful.
~Andrew
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