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

[Yicong Yang <yangyicong@huawei.com>]

Hi Ard,

After entering 6.1-rc1 the efi runtime services is not working on my platform:

[    0.054039] Remapping and enabling EFI services.
[    0.054043] UEFI virtual mapping missing or invalid -- runtime services will not be available

Not sure this patch is the root cause since I see some refactor of efi codes in 6.1-rc1,
but simply reverting this make EFI runtime services works again. Tested on HiSilicon's
Kunpeng 920 arm64 server using 48 bit VA address:

CONFIG_ARM64_VA_BITS_48=y
CONFIG_ARM64_VA_BITS=48

Thanks.

On 2022/9/16 18:18, Ard Biesheuvel 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@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;
>  }
>  
>