Re: [dpdk-dev] [PATCH] bus/pci: check if 5-level paging is enabled when testing IOMMU address width

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 10-Aug-18 9:35 AM, Drocula wrote:
> First, thanks for your suggestions.
> 
> When using the MAP_FIXED flag, mmap will return an MMAP_FAILED if
> 0xf0000000000000 is not available.
> 
> In this case, I want mmap to return an address near 0xf0000000000000.
> 
> I will submit v2.

How can we be sure there's nothing mapped at that address? I think the 
original code was correct - try mapping around that address, and see if 
we get *something* close to it with the right bits set. MAP_FIXED seems 
dangerous to use without knowing that there's nothing there. Recent 
kernels have added a safer version of MAP_FIXED, but obviously it won't 
work on the majority of kernel versions we support.

> 
> On Fri, Aug 10, 2018, 01:03 Stephen Hemminger <stephen@networkplumber.org>
> wrote:
> 
>> Thanks for the patch, there are some minor style/cleanups that
>> could be done.
>>
>>>   #if defined(RTE_ARCH_X86)
>>
>> Isn't this going to apply to 64 bit only?
>>
>>> +/*
>>> + * Try to detect whether the system uses 5-level page table.
>>> + */
>>> +static bool
>>> +system_uses_PML5(void)
>>> +{
>>> +     void *page_4k, *mask = (void *)0xf0000000000000;
>>
>> Magic constants expressed like this seem wrong. Why not use
>> shift to make it obvious.
>>
>> Also, you are assuming a particular layout of memory on
>> Linux which might be problematic. Plus if there is already
>> some memory in use there, it won't work.
>>
>>> +     page_4k = mmap(mask, 4096, PROT_READ | PROT_WRITE,
>>> +             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>>
>> Since you are probing maybe MAP_FIXED is what you want.
>>
>>> +
>>> +     if (page_4k == (void *) -1)
>>> +             return false;
>> Use MMAP_FAILED here.
>>
>>> +     munmap(page_4k, 4096);
>>> +
>>> +     if ((unsigned long)page_4k & (unsigned long)mask)
>>> +             return true;
>>> +     return false;
>>
>> Wouldn't this work the same for what you expect?
>>          return page_4k == mask;
>>
>> I.e you expect kernel to put page where you want.
>>
> 


-- 
Thanks,
Anatoly