linux-kernel - Re: [PATCH 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu

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Date:	Tue, 07 Jul 2015 16:49:50 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	david.vrabel@...rix.com, roger.pau@...rix.com,
	elena.ufimtseva@...cle.com, stefano.stabellini@...citrix.com,
	tim@....org, jbeulich@...e.com, andrew.cooper3@...rix.com,
	ian.campbell@...rix.com, wei.liu2@...rix.com,
	ian.jackson@...citrix.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init()

On 07/07/2015 04:16 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 06, 2015 at 11:34:20PM -0400, Boris Ostrovsky wrote:
>> x86-64 ABI requires that functions preserve %rbx. When
>> xen_pvh_early_cpu_init() is executed on boot cpu it is invoked as a
>> function and 'cpuid' instruction will clobber %rbx. (This is not a
>> concern on secondary processors since there xen_pvh_early_cpu_init() is
>> the entry point and thus does not need to preserve registers.)
>>
>> Since we cannot access stack on secondary processors (PTE's NX bit will
>> cause #GP on AMD --- see commit a2ef5dc2c7cb ("x86/xen: Set EFER.NX and
>> EFER.SCE in PVH guests")) we can't always save %rbx on the stack. We
>> work around this by creating a new entry point for those processors ---
>> xen_pvh_early_cpu_init_secondary(). Note that on secondary CPUs we will
>> load %rbx with garbage, which is OK.
> .. as we overwrite it with %edx.

I meant here that we do 'mov -8(%rsp), %rbx' before returning and on 
secondary CPUs we will grab garbage from the stack. But it's OK because 
we are in the entry point and cpu_bringup_and_idle (which is where we 
will be jumping to) doesn't care what's in %rbx.

Overwriting with %edx happens earlier.

-boris


>
>> As a side effect of these changes we don't need to save %rsi anymore,
>> since we can use %rbx instead of %rsi as a temporary register.
>>
>> (We could store %rbx into another scratch register such as %r8 but since
>> we will need new entry point for 32-bit PVH anyway we go with this
>> approach for symmetry)
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>
>
>> ---
>>   arch/x86/xen/smp.c      |  3 ++-
>>   arch/x86/xen/smp.h      |  4 ++++
>>   arch/x86/xen/xen-head.S | 14 +++++++-------
>>   3 files changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
>> index 8648438..ca7ee1f 100644
>> --- a/arch/x86/xen/smp.c
>> +++ b/arch/x86/xen/smp.c
>> @@ -423,7 +423,8 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
>>   		 * bit set. This means before DS/SS is touched, NX in
>>   		 * EFER must be set. Hence the following assembly glue code.
>>   		 */
>> -		ctxt->user_regs.eip = (unsigned long)xen_pvh_early_cpu_init;
>> +		ctxt->user_regs.eip =
>> +			(unsigned long)xen_pvh_early_cpu_init_secondary;
>>   		ctxt->user_regs.rdi = cpu;
>>   		ctxt->user_regs.rsi = true;  /* entry == true */
>>   	}
>> diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
>> index 963d62a..bf2b43c 100644
>> --- a/arch/x86/xen/smp.h
>> +++ b/arch/x86/xen/smp.h
>> @@ -10,10 +10,14 @@ extern void xen_send_IPI_self(int vector);
>>   
>>   #ifdef CONFIG_XEN_PVH
>>   extern void xen_pvh_early_cpu_init(int cpu, bool entry);
>> +extern void xen_pvh_early_cpu_init_secondary(int cpu, bool entry);
>>   #else
>>   static inline void xen_pvh_early_cpu_init(int cpu, bool entry)
>>   {
>>   }
>> +static inline void xen_pvh_early_cpu_init_secondary(int cpu, bool entry)
>> +{
>> +}
>>   #endif
>>   
>>   #endif
>> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
>> index 8afdfcc..b1508a8 100644
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -56,28 +56,28 @@ ENTRY(startup_xen)
>>    * @entry: true if this is a secondary vcpu coming up on this entry
>>    *         point, false if this is the boot CPU being initialized for
>>    *         the first time (%rsi)
>> - *
>> - * Note: This is called as a function on the boot CPU, and is the entry point
>> - *       on the secondary CPU.
>>    */
>>   ENTRY(xen_pvh_early_cpu_init)
>> -	mov     %rsi, %r11
>> +	mov	%rbx, -8(%rsp)
>>   
>> +/* Entry point for secondary CPUs */
>> +ENTRY(xen_pvh_early_cpu_init_secondary)
>>   	/* Gather features to see if NX implemented. */
>>   	mov     $0x80000001, %eax
>>   	cpuid
>> -	mov     %edx, %esi
>> +	mov     %edx, %ebx
>>   
>>   	mov     $MSR_EFER, %ecx
>>   	rdmsr
>>   	bts     $_EFER_SCE, %eax
>>   
>> -	bt      $20, %esi
>> +	bt      $20, %ebx
>>   	jnc     1f      	/* No NX, skip setting it */
>>   	bts     $_EFER_NX, %eax
>>   1:	wrmsr
>> +	mov	-8(%rsp), %rbx
>>   #ifdef CONFIG_SMP
>> -	cmp     $0, %r11b
>> +	cmp     $0, %esi
>>   	jne     cpu_bringup_and_idle
>>   #endif
>>   	ret
>> -- 
>> 1.8.1.4
>>

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