Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

[Andrew Cooper]

On 06/01/18 21:23, Thomas Gleixner wrote:
> On Sat, 6 Jan 2018, Andrew Cooper wrote:
>> On 06/01/18 11:49, David Woodhouse wrote:
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index 372ba3f..40e6e54 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -904,6 +904,11 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>>>  
>>>  	setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
>>>  	setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
>>> +#ifdef CONFIG_RETPOLINE
>>> +	setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
>>> +	if (c->x86_vendor == X86_VENDOR_AMD)
>>> +		setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
>> This isn't safe. It needs to be dependant on finding that LFENCEs are
>> actually dispatch serialising.
>>
>> In particular, when virtualised, you'll most likely be saddled with the
>> hypervisors choice of setting, in which case you need to use retpoline
>> as a fallback.
> On bare metal we are sure, the virtualization part is a different question.

Leaving virtualisation to one side, how does this cope with pre-SSE2
hardware?

~Andrew