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

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

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.

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