On Thu, Jun 13, 2013 at 1:47 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> 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.)
all because I update size_of_mask for old cpus that does not have
cpuid 80000008.
by make high 32bits to be all 1s. otherwise size = -mask trick will not work.
then check those range size_or_mask using, found that is not right.
as base and size could be all small, but base + size -1 could be big enough.
then the original will not detect that is out of boundary.
also we could even use x86_phys_bits directly.
>
> 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.
Yes.
also we can drop base > (base + size) checking, as
base and size are already shifted with PAGE_SHIFT to pfn.
so base+size can not be wrapped.
>
> A lot of the mask_hi/mask_lo stuff should just get removed by using
> rdmsrl/wrmsrl, but that is not stable material obviously.
yes.
will send updated version shortly.
Thanks
Yinghai
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