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

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Date:   Tue, 14 May 2019 12:26:43 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, kvm@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     konrad.wilk@...cle.com, jan.setjeeilers@...cle.com,
        liran.alon@...cle.com, jwadams@...gle.com
Subject: Re: [RFC KVM 19/27] kvm/isolation: initialize the KVM page table with
 core mappings


On 5/13/19 6:47 PM, Alexandre Chartre wrote:
> 
> 
> On 5/13/19 5:50 PM, Dave Hansen wrote:
>>> +    /*
>>> +     * Copy the mapping for all the kernel text. We copy at the PMD
>>> +     * level since the PUD is shared with the module mapping space.
>>> +     */
>>> +    rv = kvm_copy_mapping((void *)__START_KERNEL_map, KERNEL_IMAGE_SIZE,
>>> +         PGT_LEVEL_PMD);
>>> +    if (rv)
>>> +        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.