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

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Message-ID: <20180926132118.GF2555@MiWiFi-R3L-srv>
Date:   Wed, 26 Sep 2018 21:21:18 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Kairui Song <kasong@...hat.com>
Cc:     Borislav Petkov <bp@...e.de>,
        "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 09/26/18 at 01:01pm, Lendacky, Thomas wrote:
> On 09/26/2018 06:22 AM, Baoquan He wrote:
> > On 09/26/18 at 03:32pm, Baoquan He wrote:
> >> 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.
> > 
> > OK, in distros CONFIG_AMD_MEM_ENCRYPT=y is set by default usually.
> > enc_bit can save once in normal boot, then fetch and skip the cpuid
> > detection in initialize_identity_maps(). However this only speeds up in
> > amd system with SME, on intel cpu and amd cpu w/o sme, it still needs to
> > do cpuid twice. Removing it should be not measurable as Boris said.
> > Not sure if Tom has other concern.
> 
> No concern from me.  The original version of the patch did not cache the
> value, that was added based on the patch series feedback.  So, if there
> is no concern about executing some extra CPUID/RDMSR instructions, then
> it would certainly simplify the code quite a bit.

OK, thanks for confirming this, Tom.

Then, maybe Kairui can repost below code with formal patch log after
testing. I have tested on a intel machine with 48G memory, and
CONFIG_AMD_MEM_ENCRYPT=y, it works well. Maybe add Boris's Suggested-By,
and CC me.

Thanks
Baoquan

> >>
> >> 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)