Re: [Xen-devel] [PATCH] x86/XPTI: fix S3 resume (and CPU offlining in general)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.05.18 at 16:14, <simon@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Jan Beulich:
>>>>> On 24.05.18 at 16:00, <simon@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>>> Jan Beulich:
>>>> In commit d1d6fc97d6 ("x86/xpti: really hide almost all of Xen image")
>>>> I've failed to remember the fact that multiple CPUs share a stub
>>>> mapping page. Therefore it is wrong to unconditionally zap the mapping
>>>> when bringing down a CPU; it may only be unmapped when no other online
>>>> CPU uses that same page.
>>>>
>>>> Reported-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>
>>>> --- a/xen/arch/x86/smpboot.c
>>>> +++ b/xen/arch/x86/smpboot.c
>>>> @@ -876,7 +876,21 @@ static void cleanup_cpu_root_pgt(unsigne
>>>>  
>>>>      free_xen_pagetable(rpt);
>>>>  
>>>> -    /* Also zap the stub mapping for this CPU. */
>>>> +    /*
>>>> +     * Also zap the stub mapping for this CPU, if no other online one uses
>>>> +     * the same page.
>>>> +     */
>>>> +    if ( stub_linear )
>>>> +    {
>>>> +        unsigned int other;
>>>> +
>>>> +        for_each_online_cpu(other)
>>>> +            if ( !((per_cpu(stubs.addr, other) ^ stub_linear) >> 
>>>> PAGE_SHIFT) )
>>>> +            {
>>>> +                stub_linear = 0;
>>>> +                break;
>>>> +            }
>>>> +    }
>>>>      if ( stub_linear )
>>>>      {
>>>>          l3_pgentry_t *l3t = l4e_to_l3e(common_pgt);
>>>
>>> Tried this on-top of staging (fc5805daef) and I still get the same
>>> double fault.
>> 
>> Hmm, it worked for me offlining (and later re-onlining) several pCPU-s. What
>> size a system are you testing on? Mine has got only 12 CPUs, i.e. all stubs
>> are in the same page (and I'd never unmap anything here at all).
> 
> 4 cores + HT, so 8 CPUs from Xen's PoV.

May I ask you to do two things:
1) confirm that you can offline CPUs successfully using xen-hptool,
2) add a printk() to the code above making clear whether/when any
of the mappings actually get zapped?

Thanks, Jan



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