[Xen-devel] Manual differ from source code about Unrestricted Guest

[confucius <henanwxr@xxxxxxxxxxx>]

As italics show:
Some processors allow guest software to run in unpaged protected mode or in
real-address
mode, such guest called “unrestricted guest”.
If CR0.PG = 0, each linear address is passed directly to the EPT mechanism
for translation to a physical address.”
References to Intel® 64 and IA-32 Architectures Software Developer’s Manual
Volume 3B:System Programming Guide, Part 2  22.8 UNRESTRICTED GUESTS

When we set CR0.PG=0 of a guest, I think it doesn’t need construct a page
table for the unpaged guest, but I found it construct an identify_map table
for unpaged guest in the source codes of xen. As follow: 
Xen-4.0/tools/libxc/xc_hvm_build.c 
setup_guest()
{
…………………
/*
     * Identity-map page table is required for running with CR0.PG=0 when
     * using Intel EPT. Create a 32-bit non-PAE page directory of
superpages.
     */
    if ( (ident_pt = xc_map_foreign_range(
              xc_handle, dom, PAGE_SIZE, PROT_READ | PROT_WRITE,
              special_pfn(SPECIALPAGE_IDENT_PT))) == NULL )
        goto error_out;
    for ( i = 0; i < PAGE_SIZE / sizeof(*ident_pt); i++ )
        ident_pt[i] = ((i << 22) | _PAGE_PRESENT | _PAGE_RW | _PAGE_USER |
                       _PAGE_ACCESSED | _PAGE_DIRTY | _PAGE_PSE);
    munmap(ident_pt, PAGE_SIZE);
…………………
}

Why construct such identity map table for unpaged guest?
In my opinion, guset_cr3 doesn’t function when set CR0.PG=0, can guest_cr3
of this unpaged guest point to identity map table?


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