Re: [Xen-devel] [PATCH 1/4] x86/wakeup/sleep: Check whether the TSS GDT descriptor is empty before using it.

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 10/17/2012 06:49 AM, Konrad Rzeszutek Wilk wrote:
> We check the TSS descriptor before we try to dereference it.
> Also fix up the value to use the #defines.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>   arch/x86/power/cpu.c |    7 +++++--
>   1 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index 218cdb1..c17370e 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -133,7 +133,9 @@ static void fix_processor_context(void)
>   {
>         int cpu = smp_processor_id();
>         struct tss_struct *t = &per_cpu(init_tss, cpu);
> -
> +#ifdef CONFIG_X86_64
> +       struct desc_struct *desc = get_cpu_gdt_table(cpu);
> +#endif
>         set_tss_desc(cpu, t);   /*
>                                  * This just modifies memory; should not be
>                                  * necessary. But... This is necessary, because
> @@ -142,7 +144,8 @@ static void fix_processor_context(void)
>                                  */
>
>   #ifdef CONFIG_X86_64
> -       get_cpu_gdt_table(cpu)[GDT_ENTRY_TSS].type = 9;
> +       if (!desc_empty(&desc[GDT_ENTRY_TSS]))
> +               desc[GDT_ENTRY_TSS].type = DESC_TSS;
>
>         syscall_init();                         /* This sets MSR_*STAR and 
> related */
>   #endif
Why is this patch necessary?  Presumably there is something further down 
the line which depends on the TSS descriptor being empty, but if so, what?
        -hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.


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