RE: [PATCH V3 1/3] x86/mm/pageattr: Introduce helper function to unmap EFI boot services

[Prakhya, Sai Praneeth]

> Ideally, after kernel assumes control of the platform, firmware shouldn't access
> EFI boot services code/data regions. But, it's noticed that this is not so true in
> many x86 platforms. Hence, during boot, kernel reserves EFI boot services
> code/data regions [1] and maps [2] them to efi_pgd so that call to
> set_virtual_address_map() doesn't fail.
> After returning from set_virtual_address_map(), kernel frees the reserved
> regions [3] but they still remain mapped. Hence, introduce
> kernel_unmap_pages_in_pgd() which will later be used to unmap EFI boot
> services code/data regions.
> 
> While at it modify kernel_map_pages_in_pgd() by 1. Adding __init modifier
> because it's always used *only* during boot.
> 2. Add a warning if it's used after SMP is initialized because it uses
>    __flush_tlb_all() which flushes mappings only on current CPU.
> 
> Unmapping EFI boot services code/data regions will result in clearing
> PAGE_PRESENT bit and it shouldn't bother L1TF cases because it's already
> handled by protnone_mask() at arch/x86/include/asm/pgtable-invert.h.
> 
> [1] efi_reserve_boot_services()
> [2] efi_map_region() -> __map_region() -> kernel_map_pages_in_pgd() [3]
> efi_free_boot_services()
> 
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

Wasn't sure (and hence didn't) if I should add Acked-by Ingo because this patch changed from V2.

Regards,
Sai