Re: [RFC 5/7] x86/asm: Rearrange struct cpu_tss to enlarge SYSENTER_stack and fix alignment

[Dave Hansen <dave.hansen@intel.com>]

On 11/10/2017 08:05 PM, Andy Lutomirski wrote:
>  struct tss_struct {
>  	/*
> +	 * Space for the temporary SYSENTER stack.  Used for the entry
> +	 * trampoline as well.  Size it such that tss_struct ends up
> +	 * as a multiple of PAGE_SIZE.  This calculation assumes that
> +	 * io_bitmap is a multiple of PAGE_SIZE (8192 bytes) plus one
> +	 * long.
> +	 */
> +	unsigned long		SYSENTER_stack_canary;
> +	unsigned long		SYSENTER_stack[(PAGE_SIZE - sizeof(struct x86_hw_tss)) / sizeof(unsigned long) - 2];
> +
> +	/*
>  	 * The hardware state:
>  	 */
>  	struct x86_hw_tss	x86_tss;
> @@ -337,15 +347,9 @@ struct tss_struct {
>  	 * be within the limit.
>  	 */
>  	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];
> -
> -	/*
> -	 * Space for the temporary SYSENTER stack.
> -	 */
> -	unsigned long		SYSENTER_stack_canary;
> -	unsigned long		SYSENTER_stack[64];
>  } ____cacheline_aligned;


If io_bitmap[] is already page-size-aligned, how does it help us to move
SYSENTER_stack[]?

It seems like it would be easier to just leave SYSENTER_stack[] where it
is, make it SYSENTER_stack[0], and just find somewhere else to choose
how much to bloat the tss_struct *allocation* instead of trying to make
sure that sizeof(tss_struct) matches the allocation.

That SYSENTER_stack[] size calculation is pretty hideous. :)