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

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

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.

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