linux-kernel - Re: [PATCH] x86/boot: Fix kexec booting failure after SEV early boot support

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Date:   Wed, 26 Sep 2018 15:32:52 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Kairui Song <kasong@...hat.com>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "dyoung@...hat.com" <dyoung@...hat.com>
Subject: Re: [PATCH] x86/boot: Fix kexec booting failure after SEV early boot
 support

On 09/25/18 at 07:26pm, Borislav Petkov wrote:
> IINM, the problem can be addressed in a simpler way by getting rid of
> enc_bit and thus getting rid of the need to do relative addressing of
> anything and simply doing the whole dance of figuring out the C-bit each
> time. It probably wouldn't be even measurable...

Couldn't agree more.

Obviously enc_bit is redundent here. We only check eax each time,
removing it can fix the RIP-relative addressing issue in kexec.

diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index eaa843a52907..0b60eb867d25 100644
--- a/arch/x86/boot/compressed/mem_encrypt.S
+++ b/arch/x86/boot/compressed/mem_encrypt.S
@@ -27,19 +27,6 @@ ENTRY(get_sev_encryption_bit)
 	push	%edx
 	push	%edi
 
-	/*
-	 * RIP-relative addressing is needed to access the encryption bit
-	 * variable. Since we are running in 32-bit mode we need this call/pop
-	 * sequence to get the proper relative addressing.
-	 */
-	call	1f
-1:	popl	%edi
-	subl	$1b, %edi
-
-	movl	enc_bit(%edi), %eax
-	cmpl	$0, %eax
-	jge	.Lsev_exit
-
 	/* Check if running under a hypervisor */
 	movl	$1, %eax
 	cpuid
@@ -69,12 +56,10 @@ ENTRY(get_sev_encryption_bit)
 
 	movl	%ebx, %eax
 	andl	$0x3f, %eax		/* Return the encryption bit location */
-	movl	%eax, enc_bit(%edi)
 	jmp	.Lsev_exit
 
 .Lno_sev:
 	xor	%eax, %eax
-	movl	%eax, enc_bit(%edi)
 
 .Lsev_exit:
 	pop	%edi
@@ -113,9 +98,6 @@ ENTRY(set_sev_encryption_mask)
 ENDPROC(set_sev_encryption_mask)
 
 	.data
-enc_bit:
-	.int	0xffffffff
-
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	.balign	8
 GLOBAL(sme_me_mask)