[PATCH] efi/libstub: arm64: avoid SetVirtualAddressMap() when possible

[Ard Biesheuvel]

(use Peter's correct email address)

On Fri, 16 Sept 2022 at 12:19, Ard Biesheuvel <ardb at kernel.org> wrote:
>
> EFI's SetVirtualAddressMap() runtime service is a horrid hack that we'd
> like to avoid using, if possible. For 64-bit architectures such as
> arm64, the user and kernel mappings are entirely disjoint, and given
> that we use the user region for mapping the UEFI runtime regions when
> running under the OS, we don't rely on SetVirtualAddressMap() in the
> conventional way, i.e., to permit kernel mappings of the OS to coexist
> with kernel region mappings of the firmware regions. This means that, in
> principle, we should be able to avoid SetVirtualAddressMap() altogether,
> and simply use the 1:1 mapping that UEFI uses at boot time. (Note that
> omitting SetVirtualAddressMap() is explicitly permitted by the UEFI
> spec).
>
> However, there is a corner case on arm64, which, if configured for
> 3-level paging (or 2-level paging when using 64k pages), may not be able
> to cover the entire range of firmware mappings (which might contain both
> memory and MMIO peripheral mappings).
>
> So let's avoid SetVirtualAddressMap() on arm64, but only if the VA space
> is guaranteed to be of sufficient size.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  drivers/firmware/efi/libstub/arm64-stub.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index cd3bea25c762..4fff6c32899e 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -31,6 +31,15 @@ efi_status_t check_platform_features(void)
>                         efi_err("This 16 KB granular kernel is not supported by your CPU\n");
>                 return EFI_UNSUPPORTED;
>         }
> +
> +       /*
> +        * If we have 48 bits of VA space for TTBR0 mappings, we can map the
> +        * UEFI runtime regions 1:1 and so calling SetVirtualAddressMap() is
> +        * unnecessary.
> +        */
> +       if (VA_BITS_MIN >= 48)
> +               efi_novamap = true;
> +
>         return EFI_SUCCESS;
>  }
>
> --
> 2.35.1
>