Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/06/2017 10:32 AM, Josh Poimboeuf wrote:
> On Thu, Oct 05, 2017 at 04:35:03PM -0400, Boris Ostrovsky wrote:
>>>  #ifdef CONFIG_PARAVIRT
>>> +/*
>>> + * Paravirt alternatives are applied much earlier than normal alternatives.
>>> + * They are only applied when running on a hypervisor.  They replace some
>>> + * native instructions with calls to pv ops.
>>> + */
>>> +void __init apply_pv_alternatives(void)
>>> +{
>>> +   setup_force_cpu_cap(X86_FEATURE_PV_OPS);
>> Not for Xen HVM guests.
> From what I can tell, HVM guests still use pv_time_ops and
> pv_mmu_ops.exit_mmap, right?


Right, I forgot about that one.

>>> +
>>>  void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
>>>                                  struct paravirt_patch_site *end)
>>>  {
>>> diff --git a/arch/x86/kernel/cpu/hypervisor.c 
>>> b/arch/x86/kernel/cpu/hypervisor.c
>>> index 4fa90006ac68..17243fe0f5ce 100644
>>> --- a/arch/x86/kernel/cpu/hypervisor.c
>>> +++ b/arch/x86/kernel/cpu/hypervisor.c
>>> @@ -71,6 +71,8 @@ void __init init_hypervisor_platform(void)
>>>     if (!x86_hyper)
>>>             return;
>>>  
>>> +   apply_pv_alternatives();
>> Not for Xen PV guests who have already done this.
> I think it would be harmless, but yeah, it's probably best to only write
> it once.

I also wonder whether calling apply_pv_alternatives() here before
x86_hyper->init_platform() will work since the latter may be setting
those op. In fact, that's what Xen HVM does for pv_mmu_ops.exit_mmap.

-boris

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