linux-kernel - Re: [PATCH v14 2/5] x86/boot: Introduce efi_get_rsdp

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Date:   Tue, 18 Dec 2018 09:45:26 +0800
From:   Chao Fan <fanc.fnst@...fujitsu.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>, <bp@...en8.de>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
        <keescook@...omium.org>, <bhe@...hat.com>, <msys.mizuma@...il.com>,
        <indou.takao@...fujitsu.com>, <caoj.fnst@...fujitsu.com>
Subject: Re: [PATCH v14 2/5] x86/boot: Introduce efi_get_rsdp_addr() to find
 RSDP from EFI table

On Mon, Dec 17, 2018 at 06:30:32PM +0100, Ingo Molnar wrote:
>
>* Chao Fan <fanc.fnst@...fujitsu.com> wrote:
>
>> Memory information in SRAT is necessary to fix the conflict between
>> KASLR and memory-hotremove. So RSDP and SRAT should be parsed.
>> 
>> When booting form KEXEC/EFI/BIOS, the methods to compute RSDP
>> are different. When booting from EFI, EFI table points to RSDP.
>> So parse the EFI table and find the RSDP.
>> 
>> Signed-off-by: Chao Fan <fanc.fnst@...fujitsu.com>
>> ---
>>  arch/x86/boot/compressed/acpi.c | 79 +++++++++++++++++++++++++++++++++
>>  1 file changed, 79 insertions(+)
>> 
>> diff --git a/arch/x86/boot/compressed/acpi.c b/arch/x86/boot/compressed/acpi.c
>> index 44f19546c169..4151881d8713 100644
>> --- a/arch/x86/boot/compressed/acpi.c
>> +++ b/arch/x86/boot/compressed/acpi.c
>> @@ -28,3 +28,82 @@ static acpi_physical_address get_acpi_rsdp(void)
>>  #endif
>>  	return 0;
>>  }
>> +
>> +/* Search EFI table for RSDP. */
>> +static acpi_physical_address efi_get_rsdp_addr(void)
>> +{
>> +	acpi_physical_address rsdp_addr = 0;
>> +#ifdef CONFIG_EFI
>> +	efi_system_table_t *systab;
>> +	struct efi_info *e;
>
>'e' is pretty meaningless, the canonical name for efi_info local 
>variables is typically 'ei' (although this is not consistent across the 
>code).

OK, will change it.
>
>> +	bool efi_64;
>
>Is this a flag that shows whether the EFI loader is 64-bit?

Yes, I use the signature to decide the size of EFI table.
It's used here:
+	size = efi_64 ? sizeof(efi_config_table_64_t) :
+			sizeof(efi_config_table_32_t);

>
>> +	char *sig;
>> +	int size;
>> +	int i;
>> +
>> +	e = &boot_params->efi_info;
>> +	sig = (char *)&e->efi_loader_signature;
>> +
>> +	if (!strncmp(sig, EFI64_LOADER_SIGNATURE, 4)) {
>> +		efi_64 = true;
>> +	} else if (!strncmp(sig, EFI32_LOADER_SIGNATURE, 4)) {
>> +		efi_64 = false;
>> +	} else {
>> +		debug_putstr("Wrong EFI loader signature.\n");
>> +		return 0;
>> +	}
>> +
>> +	/* Get systab from boot params. Based on efi_init(). */
>> +#ifdef CONFIG_X86_64
>> +	systab = (efi_system_table_t *)(e->efi_systab | ((__u64)e->efi_systab_hi<<32));
>> +#else
>> +	if (e->efi_systab_hi || e->efi_memmap_hi) {
>> +		debug_putstr("Error getting RSDP address: EFI system table located above 4GB.\n");
>> +		return 0;
>> +	}
>> +	systab = (efi_system_table_t *)e->efi_systab;
>> +#endif
>> +
>> +	if (!systab)
>> +		return 0;
>
>Is it normal that EFI provides no 'systab'?

I think not normal, to make sure the code run normally.
We will use it below like: i < systab->nr_tables, if
systab is wrong, the code will fail to work.

>
>> +	/*
>> +	 * Get EFI tables from systab. Based on efi_config_init() and
>> +	 * efi_config_parse_tables().
>> +	 */
>> +	size = efi_64 ? sizeof(efi_config_table_64_t) :
>> +			sizeof(efi_config_table_32_t);
>> +
>> +	for (i = 0; i < systab->nr_tables; i++) {
>> +		void *config_tables;
>> +		unsigned long table;
>> +		efi_guid_t guid;
>> +
>> +		config_tables = (void *)(systab->tables + size * i);
>> +		if (efi_64) {
>> +			efi_config_table_64_t *tmp_table;
>> +
>> +			tmp_table = (efi_config_table_64_t *)config_tables;
>
>Since 'config_tables' is a void * there's no need to cast the type.
>
>> +			guid = tmp_table->guid;
>> +			table = tmp_table->table;
>> +
>> +			if (!IS_ENABLED(CONFIG_X86_64) && table >> 32) {
>> +				debug_putstr("Error getting RSDP address: EFI config table located above 4GB.\n");
>> +				return 0;
>> +			}
>> +		} else {
>> +			efi_config_table_32_t *tmp_table;
>> +
>> +			tmp_table = (efi_config_table_32_t *)config_tables;
>
>Ditto.
>
>> +			guid = tmp_table->guid;
>> +			table = tmp_table->table;
>> +		}
>
>So it looks like 
>
>> +
>> +		if (!(efi_guidcmp(guid, ACPI_TABLE_GUID)))
>> +			rsdp_addr = (acpi_physical_address)table;
>> +		else if (!(efi_guidcmp(guid, ACPI_20_TABLE_GUID)))
>> +			return (acpi_physical_address)table;
>
>'return' is not a function.

Got it, will clean the type cast.

Thanks,
Chao Fan

>
>Thanks,
>
>	Ingo
>
>