linux-kernel - Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for kexec kernel

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Message-ID: <20230705173359.GDZKWphyFbNE8id6Jm@fat_crate.local>
Date:   Wed, 5 Jul 2023 19:33:59 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tao Liu <ltao@...hat.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, ardb@...nel.org,
        linux-kernel@...r.kernel.org, bhe@...hat.com, dyoung@...hat.com,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for
 kexec kernel

On Thu, Jun 01, 2023 at 03:20:44PM +0800, Tao Liu wrote:
> A kexec kernel bootup hang is observed on Intel Atom cpu due to unmapped

s/cpu/CPU/g

> EFI config table.
> 
> Currently EFI system table is identity-mapped for the kexec kernel, but EFI
> config table is not mapped explicitly:

Why does the EFI config table *need* to be mapped explicitly?

>     commit 6bbeb276b71f ("x86/kexec: Add the EFI system tables and ACPI
>                           tables to the ident map")
> 
> Later in the following 2 commits, EFI config table will be accessed when
> enabling sev at kernel startup.

What does SEV have to do with an Intel problem?

> This may result in a page fault due to EFI
> config table's unmapped address. Since the page fault occurs at an early
> stage, it is unrecoverable and kernel hangs.
> 
>     commit ec1c66af3a30 ("x86/compressed/64: Detect/setup SEV/SME features
>                           earlier during boot")
>     commit c01fce9cef84 ("x86/compressed: Add SEV-SNP feature
>                           detection/setup")
> 
> In addition, the issue doesn't appear on all systems, because the kexec
> kernel uses Page Size Extension (PSE) for identity mapping. In most cases,
> EFI config table can end up to be mapped into due to 1 GB page size.
> However if nogbpages is set, or cpu doesn't support pdpe1gb feature
> (e.g Intel Atom x6425RE cpu), EFI config table may not be mapped into
> due to 2 MB page size, thus a page fault hang is more likely to happen.

This doesn't answer my question above.

> This patch will make sure the EFI config table is always mapped.

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.


> 
> Signed-off-by: Tao Liu <ltao@...hat.com>
> ---
> Changes in v2:
> - Rephrase the change log based on Baoquan's suggestion.
> - Rename map_efi_sys_cfg_tab() to map_efi_tables().
> - Link to v1: https://lore.kernel.org/kexec/20230525094914.23420-1-ltao@redhat.com/
> ---
>  arch/x86/kernel/machine_kexec_64.c | 35 ++++++++++++++++++++++++++----
>  1 file changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 1a3e2c05a8a5..664aefa6e896 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -28,6 +28,7 @@
>  #include <asm/setup.h>
>  #include <asm/set_memory.h>
>  #include <asm/cpu.h>
> +#include <asm/efi.h>
>  
>  #ifdef CONFIG_ACPI
>  /*
> @@ -86,10 +87,12 @@ const struct kexec_file_ops * const kexec_file_loaders[] = {
>  #endif
>  
>  static int
> -map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
> +map_efi_tables(struct x86_mapping_info *info, pgd_t *level4p)
>  {
>  #ifdef CONFIG_EFI
>  	unsigned long mstart, mend;
> +	void *kaddr;
> +	int ret;
>  
>  	if (!efi_enabled(EFI_BOOT))
>  		return 0;
> @@ -105,6 +108,30 @@ map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
>  	if (!mstart)
>  		return 0;
>  
> +	ret = kernel_ident_mapping_init(info, level4p, mstart, mend);
> +	if (ret)
> +		return ret;
> +
> +	kaddr = memremap(mstart, mend - mstart, MEMREMAP_WB);
> +	if (!kaddr) {
> +		pr_err("Could not map UEFI system table\n");
> +		return -ENOMEM;
> +	}
> +
> +	mstart = efi_config_table;

Yeah, about this, did you see efi_reuse_config() and the comment above
it especially?

Or is it that the EFI in that box wants the config table mapped 1:1 and
accesses it during boot/kexec?

In any case, this is all cloudy without a proper root cause.

Also, I'd like for Ard to have a look at this too.

Thx.

-- 
Regards/Gruss,
    Boris.

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