Re: [Xen-devel] [PATCH v8 1/9] x86/xpti: avoid copying L4 page table contents when possible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.04.18 at 10:30, <jgross@xxxxxxxx> wrote:
> @@ -160,5 +161,20 @@ unsigned int flush_area_local(const void *va, unsigned 
> int flags)
>  
>      local_irq_restore(irqfl);
>  
> +    if ( flags & FLUSH_ROOT_PGTBL )
> +        get_cpu_info()->root_pgt_changed = true;
> +
>      return flags;
>  }
> +
> +void flush_root_pgt_mask(cpumask_t *mask)

const

> +{
> +    int cpu;

unsigned int

> +    struct cpu_info *cpu_info;

Declaration in most narrow possible scope please.

> +    for_each_cpu(cpu, mask)
> +    {
> +        cpu_info = (struct cpu_info *)(stack_base[cpu] + STACK_SIZE) - 1;
> +        cpu_info->root_pgt_changed = true;
> +    }
> +}

So why is this not sending an IPI to trigger the FLUSH_ROOT_PGTBL processing
visible above? Especially the cast you need here looks quite undesirable. Plus I
think this is racy (even if quite unlikely to trigger) with a CPU being brought 
down:
What if stack_base[cpu] becomes NULL after you've found the respective bit in
mask set. Especially when Xen runs itself virtualized, (almost) arbitrarily long
periods of time may pass between any two instructions.

Jan



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