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

[Ard Biesheuvel]

On Thu, 29 Feb 2024 at 11:38, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> First
>
> Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
>
> for the patch.
>

Thanks.

> 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"...
>

Because we enter with a 1:1 mapping, and so we can only switch to
another set of page tables that also includes this 1:1 mapping. Once
we are running from the kernel mapping, we can drop the 1:1 mapping
but we still need it.

What we could do for robustness is reduce this 1:1 mapping to text +
rodata, and make it read-only, but I'm not sure it's worth the churn.

> 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