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

[Drocula <quzeyao@gmail.com>]

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.

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.
>