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

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Date:	Tue, 7 Jul 2015 16:16:26 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Boris Ostrovsky <boris.ostrovsky@...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 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.

> 
> 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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