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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan



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