Re: [Xen-devel] [PATCH v2 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init()

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Jul 16, 2015 at 05:43:36PM -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 from the stack, which is OK.
> 
> 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).
> 
> Konrad also sugested that with separate entry point for secondary
> processors we don't need the second argument ('bool entry') to
> xen_pvh_early_cpu_init[_secondary](). So it is dropped.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
> Changes in v2:
> * Dropped second argument to xen_pvh_early_cpu_init()
> 
>  arch/x86/xen/enlighten.c |  2 +-
>  arch/x86/xen/smp.c       |  4 ++--
>  arch/x86/xen/smp.h       |  8 ++++++--
>  arch/x86/xen/xen-head.S  | 27 ++++++++++++++-------------
>  4 files changed, 23 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 0b95c9b..f8dc398 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1454,7 +1454,7 @@ static void __init xen_pvh_early_guest_init(void)
>  
>       xen_have_vector_callback = 1;
>  
> -     xen_pvh_early_cpu_init(0, false);
> +     xen_pvh_early_cpu_init(0);
>       xen_pvh_set_cr_flags(0);
>  
>  #ifdef CONFIG_X86_32
> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> index 8648438..e53be3b 100644
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -423,9 +423,9 @@ 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 */
>       }
>  #endif
>       ctxt->user_regs.esp = idle->thread.sp0 - sizeof(struct pt_regs);
> diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
> index 963d62a..538c00d 100644
> --- a/arch/x86/xen/smp.h
> +++ b/arch/x86/xen/smp.h
> @@ -9,9 +9,13 @@ extern void xen_send_IPI_all(int vector);
>  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(int cpu);
> +extern void xen_pvh_early_cpu_init_secondary(int cpu);
>  #else
> -static inline void xen_pvh_early_cpu_init(int cpu, bool entry)
> +static inline void xen_pvh_early_cpu_init(int cpu)
> +{
> +}
> +static inline void xen_pvh_early_cpu_init_secondary(int cpu)
>  {
>  }
>  #endif
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 8afdfcc..944b08b 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -53,31 +53,32 @@ ENTRY(startup_xen)
>  /*
>   * xen_pvh_early_cpu_init() - early PVH VCPU initialization
>   * @cpu:   this cpu number (%rdi)
> - * @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)
> +     xor     %esi, %esi
> +     jmp     1f
> +
> +/* Entry point for secondary CPUs. Can't touch stack until NX is dealt with. 
> */
> +ENTRY(xen_pvh_early_cpu_init_secondary)
> +     mov     $1, %esi
> +1:
>       /* 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
> -     jnc     1f              /* No NX, skip setting it */
> +     bt      $20, %ebx
> +     jnc     2f              /* No NX, skip setting it */
>       bts     $_EFER_NX, %eax
> -1:   wrmsr
> +2:   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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