Re: [PATCH v6 1/8] efi: Pass KHO state to the next kernel in a configuration table

[Mukesh Pilaniya <mpilaniy@redhat.com>]

Hi George,

On 29/10/26 10:59 pm, George Guo wrote:
> From: George Guo <guodongtai@kylinos.cn>
> 
> KHO has to give the next kernel two addresses: the KHO FDT and the
> bootmem area list. arm64 and riscv put them under /chosen in the boot
> FDT. LoongArch systems booting with EFI and ACPI do not require a boot
> FDT, so KHO needs another way to pass this information.
> 
> Add a Linux EFI configuration table for them, under
> LINUX_EFI_KEXEC_HANDOVER_GUID. It points to a struct linux_efi_kho_data.
> 
>   - On a cold boot, the LoongArch EFI stub installs the table, zeroed.
>     Each kexec'd kernel is given the same EFI system table, so it finds
>     the same table.
> 
>   - In early boot, efi_config_parse_tables() reserves the table so
>     neither the page allocator nor kexec segments use it, and calls
>     kho_populate() if the table holds KHO state.
> 
>   - Provide efi_kho_update() to update the table before kexec. Map the
>     table during boot so updates do not require allocation or mapping
>     at panic time.
> 
>   - efi_kho_table_found() tells architecture code whether the table was
>     found at boot, so it can turn KHO off when the table is missing.
> 
> Only the LoongArch stub installs the table, and only with
> CONFIG_KEXEC_HANDOVER. Elsewhere there is no table and this code does
> nothing.
> 
> Signed-off-by: George Guo <guodongtai@kylinos.cn>
> ---
>  drivers/firmware/efi/efi.c               | 68 ++++++++++++++++++++++++
>  drivers/firmware/efi/libstub/efi-stub.c  | 22 ++++++++
>  drivers/firmware/efi/libstub/efistub.h   |  2 +
>  drivers/firmware/efi/libstub/loongarch.c |  2 +
>  include/linux/efi.h                      | 40 ++++++++++++++
>  5 files changed, 134 insertions(+)
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 0327a39d31fa..e409e4a57378 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -24,6 +24,7 @@
>  #include <linux/initrd.h>
>  #include <linux/io.h>
>  #include <linux/kexec.h>
> +#include <linux/kexec_handover.h>
>  #include <linux/platform_device.h>
>  #include <linux/random.h>
>  #include <linux/reboot.h>
> @@ -62,6 +63,7 @@ unsigned long __ro_after_init efi_rng_seed = EFI_INVALID_TABLE_ADDR;
>  static unsigned long __initdata mem_reserve = EFI_INVALID_TABLE_ADDR;
>  static unsigned long __initdata rt_prop = EFI_INVALID_TABLE_ADDR;
>  static unsigned long __initdata initrd = EFI_INVALID_TABLE_ADDR;
> +static unsigned long efi_kho __initdata = EFI_INVALID_TABLE_ADDR;
>  
>  extern unsigned long primary_display_table;
>  
> @@ -629,6 +631,7 @@ static const efi_config_table_type_t common_tables[] __initconst = {
>  	{EFI_TCG2_FINAL_EVENTS_TABLE_GUID,	&efi.tpm_final_log,	"TPMFinalLog"	},
>  	{EFI_CC_FINAL_EVENTS_TABLE_GUID,	&efi.tpm_final_log,	"CCFinalLog"	},
>  	{LINUX_EFI_MEMRESERVE_TABLE_GUID,	&mem_reserve,		"MEMRESERVE"	},
> +	{LINUX_EFI_KEXEC_HANDOVER_GUID,		&efi_kho,		"KHO"		},
>  	{LINUX_EFI_INITRD_MEDIA_GUID,		&initrd,		"INITRD"	},
>  	{EFI_RT_PROPERTIES_TABLE_GUID,		&rt_prop,		"RTPROP"	},
>  #ifdef CONFIG_OVMF_DEBUG_LOG
> @@ -806,6 +809,23 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
>  		}
>  	}
>  
> +	if (efi_kho != EFI_INVALID_TABLE_ADDR) {
> +		struct linux_efi_kho_data *kho;
> +
> +		/* Keep the buddy allocator and kexec segments off the table. */
> +		memblock_reserve(efi_kho, sizeof(*kho));
> +
> +		kho = early_memremap(efi_kho, sizeof(*kho));
> +		if (kho) {
> +			if (kho->fdt_addr)
> +				kho_populate((phys_addr_t)kho->fdt_addr, kho->fdt_size,
> +					     (phys_addr_t)kho->bootmem_addr, kho->bootmem_size);
> +			early_memunmap(kho, sizeof(*kho));
> +		} else {
> +			pr_warn("Could not map UEFI KHO table!\n");
This is a Linux EFI configuration table, not a __UEFI KHO table__, so
IMHO; UEFI wording is wrong here.

> +		}
> +	}
> +
>  	if (rt_prop != EFI_INVALID_TABLE_ADDR) {
>  		efi_rt_properties_table_t *tbl;
>  
> @@ -1171,6 +1191,54 @@ static int __init efi_memreserve_root_init(void)
>  }
>  early_initcall(efi_memreserve_root_init);
>  
> +static struct linux_efi_kho_data *efi_kho_table __ro_after_init;
> +
> +/* Did the previous kernel or the EFI stub pass a KHO table? */
> +bool __init efi_kho_table_found(void)
> +{
> +	return efi_kho != EFI_INVALID_TABLE_ADDR;
> +}
> +
> +static int __init efi_kho_table_init(void)
> +{
> +	if (efi_kho == EFI_INVALID_TABLE_ADDR)
> +		return 0;
> +
> +	/* Map it now: the table is also written at panic time. */
> +	efi_kho_table = memremap(efi_kho, sizeof(*efi_kho_table), MEMREMAP_WB);
> +	if (WARN_ON_ONCE(!efi_kho_table))
> +		efi_kho_table = ERR_PTR(-ENOMEM);
> +
> +	return 0;
> +}
> +early_initcall(efi_kho_table_init);
> +
> +/*
> + * Write the KHO addresses the next kernel should use, or zeroes
> + * if it should boot without KHO.
> + */
> +int efi_kho_update(phys_addr_t fdt_addr, u64 fdt_size,
> +		   phys_addr_t bootmem_addr, u64 bootmem_size)
> +{
> +	struct linux_efi_kho_data *kho = efi_kho_table;
> +
> +	/*
> +	 * No table: either the architecture passes the KHO state some other
> +	 * way, or it turned KHO off at boot because the table was missing.
> +	 */
> +	if (!kho)
> +		return 0;
> +	if (IS_ERR(kho))
> +		return PTR_ERR(kho);
> +
> +	kho->fdt_addr		= fdt_addr;
> +	kho->fdt_size		= fdt_size;
> +	kho->bootmem_addr	= bootmem_addr;
> +	kho->bootmem_size	= bootmem_size;
> +
> +	return 0;
> +}
> +
>  #ifdef CONFIG_KEXEC
>  static int update_efi_random_seed(struct notifier_block *nb,
>  				  unsigned long code, void *unused)
> diff --git a/drivers/firmware/efi/libstub/efi-stub.c b/drivers/firmware/efi/libstub/efi-stub.c
> index 42d6073bcd06..0fb0d4041d1c 100644
> --- a/drivers/firmware/efi/libstub/efi-stub.c
> +++ b/drivers/firmware/efi/libstub/efi-stub.c
> @@ -100,6 +100,28 @@ static void install_memreserve_table(void)
>  		efi_err("Failed to install memreserve config table!\n");
>  }
>  
> +void install_kho_table(void)
> +{
> +	struct linux_efi_kho_data *kho;
> +	efi_guid_t kho_table_guid = LINUX_EFI_KEXEC_HANDOVER_GUID;
> +	efi_status_t status;
> +
> +	if (!IS_ENABLED(CONFIG_KEXEC_HANDOVER))
> +		return;
> +
> +	status = efi_bs_call(allocate_pool, EFI_LOADER_DATA, sizeof(*kho), (void **)&kho);
> +	if (status != EFI_SUCCESS) {
> +		efi_err("Failed to allocate KHO config table!\n");
> +		return;
> +	}
> +
> +	*kho = (struct linux_efi_kho_data){};
> +
> +	status = efi_bs_call(install_configuration_table, &kho_table_guid, kho);

> +	if (status != EFI_SUCCESS)
> +		efi_err("Failed to install KHO config table!\n");
When install_configuration_table fails, the pool allocation made
just above using efi_bs_callis never freed, so a page of
EFI_LOADER_DATA is leaked until ExitBootServices, This is a minor leak
so mentioned here for review completeness.

> +}
> +
>  static u32 get_supported_rt_services(void)
>  {
>  	const efi_rt_properties_table_t *rt_prop_table;
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index fd91fc15ec81..9e0034c1260a 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -1057,6 +1057,8 @@ efi_status_t efi_exit_boot_services(void *handle, void *priv,
>  efi_status_t efi_boot_kernel(void *handle, efi_loaded_image_t *image,
>  			     unsigned long kernel_addr, char *cmdline_ptr);
>  
> +void install_kho_table(void);
> +
>  void *get_fdt(unsigned long *fdt_size);
>  
>  efi_status_t efi_alloc_virtmap(efi_memory_desc_t **virtmap,
> diff --git a/drivers/firmware/efi/libstub/loongarch.c b/drivers/firmware/efi/libstub/loongarch.c
> index 2b0c87dc9908..c2df097771e4 100644
> --- a/drivers/firmware/efi/libstub/loongarch.c
> +++ b/drivers/firmware/efi/libstub/loongarch.c
> @@ -74,6 +74,8 @@ efi_status_t efi_boot_kernel(void *handle, efi_loaded_image_t *image,
>  	efi_status_t status;
>  	u32 desc_ver;
>  
> +	install_kho_table();
> +
>  	status = efi_alloc_virtmap(&priv.runtime_map, &desc_size, &desc_ver);
>  	if (status != EFI_SUCCESS) {
>  		efi_err("Unable to retrieve UEFI memory map.\n");
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index aa15ff88539b..253e98bf9922 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -422,6 +422,7 @@ void efi_native_runtime_setup(void);
>  #define LINUX_EFI_COCO_SECRET_AREA_GUID		EFI_GUID(0xadf956ad, 0xe98c, 0x484c,  0xae, 0x11, 0xb5, 0x1c, 0x7d, 0x33, 0x64, 0x47)
>  #define LINUX_EFI_BOOT_MEMMAP_GUID		EFI_GUID(0x800f683f, 0xd08b, 0x423a,  0xa2, 0x93, 0x96, 0x5c, 0x3c, 0x6f, 0xe2, 0xb4)
>  #define LINUX_EFI_UNACCEPTED_MEM_TABLE_GUID	EFI_GUID(0xd5d1de3c, 0x105c, 0x44f9,  0x9e, 0xa9, 0xbc, 0xef, 0x98, 0x12, 0x00, 0x31)
> +#define LINUX_EFI_KEXEC_HANDOVER_GUID		EFI_GUID(0xc941b6c7, 0x7b3f, 0x4af6,  0x9e, 0x50, 0xfc, 0xb3, 0xa8, 0x86, 0x8a, 0x17)
>  
>  #define RISCV_EFI_BOOT_PROTOCOL_GUID		EFI_GUID(0xccd15fec, 0x6f73, 0x4eec,  0x83, 0x95, 0x3e, 0x69, 0xe4, 0xb9, 0x40, 0xbf)
>  
> @@ -1273,6 +1274,45 @@ struct linux_efi_memreserve {
>  
>  void __init efi_arch_mem_reserve(phys_addr_t addr, u64 size);
>  
> +/**
> + * struct linux_efi_kho_data - KHO state passed to the next kernel
> + * @fdt_addr: physical address of the KHO FDT, 0 if there is none
> + * @fdt_size: size of the KHO FDT
> + * @bootmem_addr: physical address of the bootmem area list
> + * @bootmem_size: size of the bootmem area list
> + *
> + * Pointed to by the LINUX_EFI_KEXEC_HANDOVER_GUID configuration table.
> + *
> + * The EFI stub installs this table zeroed on a cold boot. It stays
> + * in place across kexec; the running kernel rewrites it just before
> + * each kexec, and the next kernel reads it during early boot.
This statement is only true for the LoongArch and on arm64/riscv/x86 no
table exists, so the comment describes a kernel-wide invariant that does
not hold.

> + *
> + * The layout is fixed. Changing it requires a new GUID.
> + */
> +struct linux_efi_kho_data {
> +	u64 fdt_addr;
> +	u64 fdt_size;
> +	u64 bootmem_addr;
> +	u64 bootmem_size;
> +};
> +
> +#ifdef CONFIG_EFI
> +bool efi_kho_table_found(void);
> +int efi_kho_update(phys_addr_t fdt_addr, u64 fdt_size,
> +		   phys_addr_t bootmem_addr, u64 bootmem_size);
> +#else
> +static inline bool efi_kho_table_found(void)
> +{
> +	return false;
> +}
> +
> +static inline int efi_kho_update(phys_addr_t fdt_addr, u64 fdt_size,
> +				 phys_addr_t bootmem_addr, u64 bootmem_size)
> +{
> +	return 0;
> +}
> +#endif
> +
>  /*
>   * The LINUX_EFI_MOK_VARIABLE_TABLE_GUID config table can be provided
>   * to the kernel by an EFI boot loader. The table contains a packed

-- 
Regards,
Mukesh Pilaniya