Re: [PATCH v2 2/7] ARM: virt: allow the kernel to be entered in HYP mode

From: Marc Zyngier <marc.zyngier@arm.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Dave Martin <dave.martin@linaro.org>,
	linux-omap@vger.kernel.org,
	Russell King - ARM Linux <linux@arm.linux.org.uk>,
	linux-arm-kernel@lists.infradead.org,
	Nicolas Pitre <nico@fluxnic.net>
Subject: Re: [PATCH v2 2/7] ARM: virt: allow the kernel to be entered in HYP mode
Date: Sat, 06 Oct 2012 12:18:23 +0200	[thread overview]
Message-ID: <6d3553bfe8a2d1ac88cab852100616a7@localhost> (raw)
In-Reply-To: <20121005200822.GQ3874@atomide.com>

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Hi Tony,

On Fri, 5 Oct 2012 13:08:22 -0700, Tony Lindgren <tony@atomide.com> wrote:
> Hi,
> 
> * Marc Zyngier <marc.zyngier@arm.com> [120907 10:04]:
>> From: Dave Martin <dave.martin@linaro.org>
>> 
>> This patch does two things:
>> 
>>   * Ensure that asynchronous aborts are masked at kernel entry.
>>     The bootloader should be masking these anyway, but this reduces
>>     the damage window just in case it doesn't.
>> 
>>   * Enter svc mode via exception return to ensure that CPU state is
>>     properly serialised.  This does not matter when switching from
>>     an ordinary privileged mode ("PL1" modes in ARMv7-AR rev C
>>     parlance), but it potentially does matter when switching from a
>>     another privileged mode such as hyp mode.
>> 
>> This should allow the kernel to boot safely either from svc mode or
>> hyp mode, even if no support for use of the ARM Virtualization
>> Extensions is built into the kernel.
>> 
>> Signed-off-by: Dave Martin <dave.martin@linaro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> 
> Just bisected this down in linux-next for breaking booting of
> my omap2420 ARMv6 based n8x0..
> 
>> --- a/arch/arm/kernel/head.S
>> +++ b/arch/arm/kernel/head.S
>> @@ -83,8 +83,12 @@ ENTRY(stext)
>>   THUMB(	.thumb			)	@ switch to Thumb now.
>>   THUMB(1:			)
>>  
>> -	setmode	PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
>> -						@ and irqs disabled
>> +#ifdef CONFIG_ARM_VIRT_EXT
>> +	bl	__hyp_stub_install
>> +#endif
>> +	@ ensure svc mode and all interrupts masked
>> +	safe_svcmode_maskall r9
>> +
>>  	mrc	p15, 0, r9, c0, c0		@ get processor id
>>  	bl	__lookup_processor_type		@ r5=procinfo r9=cpuid
>>  	movs	r10, r5				@ invalid processor (r5=0)?
> 
> ..and looks like undoing this part fixes it. Any ideas?
> 
> I quickly tried disabling ARCH_OMAP3 and ARCH_OMAP4 so it's
> ARMv6 but that does not help.

If you compiled for v6 only, we can safely exclude __hyp_stub_install, and
I assume that you get past the decompressor.

If so, that indicates some side effect of the safe_svcmode_maskall macro,
and I suspect the "movs pc, lr" bit.

Can you try the attached patch? It basically falls back to the previous
behaviour if not entered in HYP mode.

Thanks,

        M.
-- 
Fast, cheap, reliable. Pick two.

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diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 658a15d..b21b97f 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -254,16 +254,17 @@
 	mov	lr , \reg
 	and	lr , lr , #MODE_MASK
 	cmp	lr , #HYP_MODE
-	orr	\reg , \reg , #PSR_A_BIT | PSR_I_BIT | PSR_F_BIT
+	orr	\reg , \reg , #PSR_I_BIT | PSR_F_BIT
 	bic	\reg , \reg , #MODE_MASK
 	orr	\reg , \reg , #SVC_MODE
 THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
-	msr	spsr_cxsf, \reg
-	adr	lr, BSYM(2f)
 	bne	1f
+	orr	\reg, \reg, #PSR_A_BIT
+	adr	lr, BSYM(2f)
+	msr	spsr_cxsf, \reg
 	__MSR_ELR_HYP(14)
 	__ERET
-1:	movs	pc, lr
+1:	msr	cpsr_c, \reg
 2:
 .endm
 

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