Re: [PATCH 3/3] mini-os: fix number of pages for PVH

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Juergen Gross, le sam. 18 juin 2022 16:07:07 +0200, a ecrit:
> On 18.06.22 14:13, Samuel Thibault wrote:
> > Hello,
> > 
> > Juergen Gross, le sam. 18 juin 2022 12:48:16 +0200, a ecrit:
> > > @@ -124,7 +126,7 @@ void arch_mm_preinit(void *p)
> > >           do_exit();
> > >       }
> > > -    last_free_pfn = e820_get_maxpfn(ret);
> > > +    last_free_pfn = e820_get_maxpfn(ret - e820_initial_reserved_pfns);
> > 
> > Mmm, but the reserved pfn could be in the middle of the e820 address
> > space.
> 
> That doesn't matter.
> 
> e820_get_maxpfn(n) will just return the pfn of the n-th RAM pfn it is
> finding in the E820 map.

Yes, but subtracting at this point looks a bit hacky to me.

It seems to me that it'd be better to make e820_get_maxpfn count by
itself the reserved pages (but never return its pfn of course), rather
than having to make e820_sanitize look at the reserved pages, store
it somewhere, and hope that other code will remember to subtract that
before calling e820_get_maxpfn.

I mean something like:

unsigned long e820_get_maxpfn(unsigned long pages)
{
    int i;
    unsigned long pfns = 0, start = 0;

    if ( !e820_entries )
        e820_get_memmap();

    for ( i = 0; i < e820_entries; i++ )
    {
        pfns = e820_map[i].size >> PAGE_SHIFT;

        if ( e820_map[i].type == E820_RESERVED )
        {
            /* This counts in the memory reservation, but is not usable */
            pages -= pfns;
            continue;
        }
        if ( e820_map[i].type != E820_RAM )
            continue;

        start = e820_map[i].addr >> PAGE_SHIFT;
        if ( pages <= pfns )
            return start + pages;
        pages -= pfns;
    }

    return start + pfns;
}

Samuel