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Date: Fri, 17 Jul 2015 11:09:50 -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, ian.jackson@...citrix.com,
xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init()
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@...cle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
> 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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