linux-kernel - Re: [PATCH v15 4/6] x86/boot: Introduce bios_get_rsdp

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Date:   Thu, 10 Jan 2019 22:27:47 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Chao Fan <fanc.fnst@...fujitsu.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, 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 v15 4/6] x86/boot: Introduce bios_get_rsdp_addr() to
 search RSDP in memory

On Mon, Jan 07, 2019 at 11:22:41AM +0800, Chao Fan wrote:
> Memory information in SRAT table 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 BIOS, there is no variable who can
> point to RSDP directly, so scan memory for the RSDP and verify RSDP
> by signature and checksum.
> 
> Signed-off-by: Chao Fan <fanc.fnst@...fujitsu.com>
> ---
>  arch/x86/boot/compressed/acpi.c | 86 +++++++++++++++++++++++++++++++++
>  1 file changed, 86 insertions(+)

...

> +/* Search RSDP address, based on acpi_find_root_pointer(). */
> +static acpi_physical_address bios_get_rsdp_addr(void)
> +{
> +	u8 *table_ptr;
> +	u32 address;
> +	u8 *rsdp;

But those u8's together:

	u8 *table_ptr, *rsdp;
	u32 address;

> +
> +	/* Get the location of the Extended BIOS Data Area (EBDA) */
> +	table_ptr = (u8 *)ACPI_EBDA_PTR_LOCATION;
> +	address = *(u16 *)table_ptr;
> +	address <<= 4;
> +	table_ptr = (u8 *)(long)address;
> +
> +	/*
> +	 * Search EBDA paragraphs (EBDA is required to be a minimum of
> +	 * 1K length)
> +	 */
> +	if (address > 0x400) {
> +		rsdp = scan_mem_for_rsdp(table_ptr, ACPI_EBDA_WINDOW_SIZE);
> +		if (rsdp) {
> +			address += (u32)ACPI_PTR_DIFF(rsdp, table_ptr);
> +			return address;
> +		}
> +	}
> +
> +	/* Search upper memory: 16-byte boundaries in E0000h-FFFFFh */
> +	table_ptr = (u8 *)ACPI_HI_RSDP_WINDOW_BASE;
> +	rsdp = scan_mem_for_rsdp(table_ptr, ACPI_HI_RSDP_WINDOW_SIZE);
> +

Superfluous newline.

> +	if (rsdp) {
> +		address = (u32)(ACPI_HI_RSDP_WINDOW_BASE +
> +				ACPI_PTR_DIFF(rsdp, table_ptr));
> +		return address;
> +	}
> +	return 0;
> +}

-- 
Regards/Gruss,
    Boris.

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