Re: [RFC KVM 19/27] kvm/isolation: initialize the KVM page table with core mappings

[Alexandre Chartre]

On 5/13/19 6:47 PM, Alexandre Chartre wrote:
> On 5/13/19 5:50 PM, Dave Hansen wrote:
> > > +ÂÂÂ /*
> > > +ÂÂÂÂ * Copy the mapping for all the kernel text. We copy at the PMD
> > > +ÂÂÂÂ * level since the PUD is shared with the module mapping space.
> > > +ÂÂÂÂ */
> > > +ÂÂÂ rv = kvm_copy_mapping((void *)__START_KERNEL_map, KERNEL_IMAGE_SIZE,
> > > +ÂÂÂÂÂÂÂÂ PGT_LEVEL_PMD);
> > > +ÂÂÂ if (rv)
> > > +ÂÂÂÂÂÂÂ goto out_uninit_page_table;
> >
> > Could you double-check this? We (I) have had some repeated confusion
> > with the PTI code and kernel text vs. kernel data vs. __init.
> > KERNEL_IMAGE_SIZE looks to be 512MB which is quite a bit bigger than
> > kernel text.
>
> I probably have the same confusion :-) but I will try to check again.

mm.txt says that kernel text is 512MB, and that's probably why I used
KERNEL_IMAGE_SIZE.

https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt

========================================================================================================================
    Start addr    |   Offset   |     End addr     |  Size   | VM area description
========================================================================================================================
 [...]
 ffffffff80000000 |   -2    GB | ffffffff9fffffff |  512 MB | kernel text mapping, mapped to physical address 0
 [...]


However, vmlinux.lds.S does:

. = ASSERT((_end - _text <= KERNEL_IMAGE_SIZE),
           "kernel image bigger than KERNEL_IMAGE_SIZE");

So this covers everything between _text and _end, which includes text, data,
init and other stuff

The end of the text section is tagged with _etext. So the text section is
effectively (_etext - _text). This matches with what efi_setup_page_tables()
used to copy kernel text:

int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
{
	[...]
        npages = (_etext - _text) >> PAGE_SHIFT;
        text = __pa(_text);
        pfn = text >> PAGE_SHIFT;

        pf = _PAGE_RW | _PAGE_ENC;
        if (kernel_map_pages_in_pgd(pgd, pfn, text, npages, pf)) {
                pr_err("Failed to map kernel text 1:1\n");
                return 1;
        }
	[...]
}


alex.