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SubjectRe: [PATCH 3/7] x86/boot/compressed/64: Setup IDT in startup_32 boot path
On Wed, Feb 10, 2021 at 11:21:31AM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> This boot path needs exception handling when it is used with SEV-ES.

For ?

Let's explain pls.

> Setup an IDT and provide a helper function to write IDT entries for
> use in 32-bit protected mode.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/boot/compressed/head_64.S | 73 +++++++++++++++++++++++++++++-
> 1 file changed, 72 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index c59c80ca546d..8deeec78cdb4 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -116,6 +116,11 @@ SYM_FUNC_START(startup_32)
> lretl
> 1:
>
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> + /* Setup Exception handling for SEV-ES */
> + call startup32_load_idt
> +#endif
> +

You can push that ifdeffery out of the main path (diff ontop):

---
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 8deeec78cdb4..cb5a6849fb29 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -116,10 +116,7 @@ SYM_FUNC_START(startup_32)
lretl
1:

-#ifdef CONFIG_AMD_MEM_ENCRYPT
- /* Setup Exception handling for SEV-ES */
call startup32_load_idt
-#endif

/* Make sure cpu supports long mode. */
call verify_cpu
@@ -854,16 +851,18 @@ SYM_FUNC_START(startup32_set_idt_entry)
pop %ebx
ret
SYM_FUNC_END(startup32_set_idt_entry)
+#endif

+/* Setup Exception handling for SEV-ES */
SYM_FUNC_START(startup32_load_idt)
+#ifdef CONFIG_AMD_MEM_ENCRYPT
/* Load IDT */
leal rva(boot32_idt)(%ebp), %eax
movl %eax, rva(boot32_idt_desc+2)(%ebp)
lidt rva(boot32_idt_desc)(%ebp)
-
+#endif
ret
SYM_FUNC_END(startup32_load_idt)
-#endif
/*
* Stack and heap for uncompression
*/
---
> +SYM_FUNC_START(startup32_set_idt_entry)
> + push %ebx
> + push %ecx
> +
> + /* IDT entry address to %ebx */
> + leal rva(boot32_idt)(%ebp), %ebx
> + shl $3, %edx
> + addl %edx, %ebx
> +
> + /* Build IDT entry, lower 4 bytes */
> + movl %eax, %edx

Let's add some side comments here:

+ andl $0x0000ffff, %edx # Target code segment offset [15:0]

+ movl $__KERNEL32_CS, %ecx # Target code segment selector
> + shl $16, %ecx
> + orl %ecx, %edx
> +
> + /* Store lower 4 bytes to IDT */
> + movl %edx, (%ebx)
> +
> + /* Build IDT entry, upper 4 bytes */
> + movl %eax, %edx
+ andl $0xffff0000, %edx # Target code segment offset [31:16]
+ orl $0x00008e00, %edx # Present, Type 32-bit Interrupt Gate

so that a reader like me can quickly find interrupt gates in the docs.

Thx.

--
Regards/Gruss,
Boris.

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

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