linux-kernel - Re: [PATCH 3/7] x86/boot/compressed/64: Setup IDT in startup

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Message-ID: <20210224104952.GA20344@zn.tnic>
Date:   Wed, 24 Feb 2021 11:49:52 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [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@...e.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@...e.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.

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