Re: [PATCH v7 4/9] x86/startup_64: Simplify virtual switch on primary boot

[Borislav Petkov]

First

Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>

for the patch.

On Tue, Feb 27, 2024 at 04:19:12PM +0100, Ard Biesheuvel wrote:
> +	/*
> +	 * Switch to early_top_pgt which still has the identity mappings
> +	 * present.

I was wondering why we've had this "discrepancy" forever - the boot CPU
would have early_top_pgt *with* the ident mappings while the APs would do
init_top_pgt.

But we end up loading init_top_pgt on the BSP too in init_mem_mapping()
so there's a short time during boot where we have this difference.
I haven't found a reason to have it yet except "why bother"...

And now some details just for future reference:

On the BSP:

=> 0x10000a0:   mov    %rax,%cr3

cr3            0x9922000
111850: ffffffff89922000  8192 OBJECT  GLOBAL DEFAULT   22 early_top_pgt

(gdb) p/x early_top_pgt
$3 = {{pgd = 0x9924063}, {pgd = 0x9924063}, {pgd = 0x0} <repeats 509 times>, {pgd = 0x2418067}}

first two PGDs and the last one are populated.

On the AP:

cr3            0x2416000
104747: ffffffff82416000  8192 OBJECT  GLOBAL DEFAULT   12 init_top_pgt

(gdb) p/x (long[512])*0xffffffff82416000
$8 = {0x0 <repeats 273 times>, 0xbe01067, 0x0 <repeats 128 times>, 0xc000067, 0xc001067, 0xc002067, 0xc003067, 0xc004067, 0xc005067,
  0xc006067, 0xc007067, 0xc008067, 0xc009067, 0xc00a067, 0xc00b067, 0xc00c067, 0xc00d067, 0xc00e067, 0xc00f067, 0xc010067, 0xc011067,
  0xc012067, 0xc013067, 0xc014067, 0xc015067, 0xc016067, 0xc017067, 0xc018067, 0xc019067, 0xc01a067, 0xc01b067, 0xc01c067, 0xc01d067,
  0xc01e067, 0xc01f067, 0xc020067, 0xc021067, 0xc022067, 0xc023067, 0xc024067, 0xc025067, 0xc026067, 0xc027067, 0xc028067, 0xc029067,
  0xc02a067, 0xc02b067, 0xc02c067, 0xc02d067, 0xc02e067, 0xc02f067, 0xc030067, 0xc031067, 0xc032067, 0xc033067, 0xc034067, 0xc035067,
  0xc036067, 0xc037067, 0xc038067, 0xc039067, 0xc03a067, 0xc03b067, 0xc03c067, 0xc03d067, 0xc03e067, 0xc03f067, 0x0, 0x0, 0x7ffd3067,
  0x0 <repeats 39 times>, 0x7ffd1067, 0x0, 0x9b11067, 0x2418067}

and that one becomes the swapper_pg_dir which is the kernel pagetable we
use.

PTI then does two separate ones, which is a whole different topic.

:-)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette