Re: [PATCH] x86: Fix /proc/mtrr with base/size more than 44bits

["H. Peter Anvin" <hpa@zytor.com>]

On 06/13/2013 11:53 AM, Yinghai Lu wrote:
>  
> -	if (base & size_or_mask || size & size_or_mask) {
> +	if (base >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT) ||
> +	    base > (base + size) ||
> +	    (base + size - 1) >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT)) {
>  		pr_warning("mtrr: base or size exceeds the MTRR width\n");
>  		return -EINVAL;
>  	}

Most of this patch looks good as far as being a minimal patch, but I'm
really confused about this bit.  Could you explain the reason for why
the original doesn't work?  (To be fair: I am not even sure the original
does anything useful so it could just be a "this is just too broken to
live" kind of thing.)

The first and third clause of the test can be simplified, however:

	(base | (base + size - 1)) >> (boot_cpu_data.x86_phys_bits - PAGE_SHIFT)

... although it would be cleaner to put boot_cpu_data.x86_phys_bits -
PAGE_SHIFT into a variable.

A lot of the mask_hi/mask_lo stuff should just get removed by using
rdmsrl/wrmsrl, but that is not stable material obviously.

	-hpa