Re: [Xen-devel] [V5 PATCH 1/1] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[Konrad Rzeszutek Wilk]

On Wed, Sep 10, 2014 at 04:36:06PM -0700, Mukesh Rathor wrote:
> This fixes two bugs in PVH guests:
> 
>   - Not setting EFER.NX means the NX bit in page table entries is
>     ignored on Intel processors and causes reserved bit page faults on
>     AMD processors.
> 
>   - After the Xen commit 7645640d6ff1 ("x86/PVH: don't set EFER_SCE for
>     pvh guest") PVH guests are required to set EFER.SCE to enable the
>     SYSCALL instruction.
> 
> Secondary VCPUs are started with pagetables with the NX bit set so
> EFER.NX must be set before using any stack or data segment.
> xen_pvh_cpu_early_init() is the new secondary VCPU entry point that
> sets EFER before jumping to cpu_bringup_and_idle().
> 
> Signed-off-by: Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Huh? So who wrote it? Or did you mean 'Reviewed-by'?

> ---
>  arch/x86/xen/enlighten.c |  6 ++++++
>  arch/x86/xen/smp.c       | 29 ++++++++++++++++++-----------
>  arch/x86/xen/smp.h       |  8 ++++++++
>  arch/x86/xen/xen-head.S  | 33 +++++++++++++++++++++++++++++++++
>  4 files changed, 65 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index c0cb11f..424d831 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1463,6 +1463,7 @@ static void __ref xen_setup_gdt(int cpu)
>  	pv_cpu_ops.load_gdt = xen_load_gdt;
>  }
>  
> +#ifdef CONFIG_XEN_PVH
>  /*
>   * A PV guest starts with default flags that are not set for PVH, set them
>   * here asap.
> @@ -1508,12 +1509,15 @@ static void __init xen_pvh_early_guest_init(void)
>  		return;
>  
>  	xen_have_vector_callback = 1;
> +
> +	xen_pvh_early_cpu_init(0, false);
>  	xen_pvh_set_cr_flags(0);
>  
>  #ifdef CONFIG_X86_32
>  	BUG(); /* PVH: Implement proper support. */
>  #endif
>  }
> +#endif    /* CONFIG_XEN_PVH */
>  
>  /* First C function to be called on Xen boot */
>  asmlinkage __visible void __init xen_start_kernel(void)
> @@ -1527,7 +1531,9 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  	xen_domain_type = XEN_PV_DOMAIN;
>  
>  	xen_setup_features();
> +#ifdef CONFIG_XEN_PVH
>  	xen_pvh_early_guest_init();
> +#endif
>  	xen_setup_machphys_mapping();
>  
>  	/* Install Xen paravirt ops */
> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> index 7005974..b25f8942 100644
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -37,6 +37,7 @@
>  #include <xen/hvc-console.h>
>  #include "xen-ops.h"
>  #include "mmu.h"
> +#include "smp.h"
>  
>  cpumask_var_t xen_cpu_initialized_map;
>  
> @@ -99,10 +100,14 @@ static void cpu_bringup(void)
>  	wmb();			/* make sure everything is out */
>  }
>  
> -/* Note: cpu parameter is only relevant for PVH */
> -static void cpu_bringup_and_idle(int cpu)
> +/*
> + * Note: cpu parameter is only relevant for PVH. The reason for passing it
> + * is we can't do smp_processor_id until the percpu segments are loaded, for
> + * which we need the cpu number! So we pass it in rdi as first parameter.
> + */

Thank you for expanding on that (I totally forgot why we did that).

> +asmlinkage __visible void cpu_bringup_and_idle(int cpu)
>  {
> -#ifdef CONFIG_X86_64
> +#ifdef CONFIG_XEN_PVH
>  	if (xen_feature(XENFEAT_auto_translated_physmap) &&
>  	    xen_feature(XENFEAT_supervisor_mode_kernel))
>  		xen_pvh_secondary_vcpu_init(cpu);
> @@ -374,11 +379,10 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
>  	ctxt->user_regs.fs = __KERNEL_PERCPU;
>  	ctxt->user_regs.gs = __KERNEL_STACK_CANARY;
>  #endif
> -	ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
> -
>  	memset(&ctxt->fpu_ctxt, 0, sizeof(ctxt->fpu_ctxt));
>  
>  	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> +		ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
>  		ctxt->flags = VGCF_IN_KERNEL;
>  		ctxt->user_regs.eflags = 0x1000; /* IOPL_RING1 */
>  		ctxt->user_regs.ds = __USER_DS;
> @@ -413,15 +417,18 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
>  					(unsigned long)xen_failsafe_callback;
>  		ctxt->user_regs.cs = __KERNEL_CS;
>  		per_cpu(xen_cr3, cpu) = __pa(swapper_pg_dir);
> -#ifdef CONFIG_X86_32
>  	}
> -#else
> -	} else
> -		/* N.B. The user_regs.eip (cpu_bringup_and_idle) is called with
> -		 * %rdi having the cpu number - which means are passing in
> -		 * as the first parameter the cpu. Subtle!
> +#ifdef CONFIG_XEN_PVH
> +	else {
> +		/*
> +		 * The vcpu comes on kernel page tables which have the NX pte
> +		 * bit set. This means before DS/SS is touched, NX in
> +		 * EFER must be set. Hence the following assembly glue code.

And you ripped out the nice 'N.B' comment I added. Sad :-(
>  		 */
> +		ctxt->user_regs.eip = (unsigned long)xen_pvh_early_cpu_init;
>  		ctxt->user_regs.rdi = cpu;
> +		ctxt->user_regs.rsi = true;  /* secondary cpu == true */

Oh, that is new. Ah yes we can use that [looking at Xen code].
I wonder what other registers we can use to pass stuff around.


> +	}
>  #endif
>  	ctxt->user_regs.esp = idle->thread.sp0 - sizeof(struct pt_regs);
>  	ctxt->ctrlreg[3] = xen_pfn_to_cr3(virt_to_mfn(swapper_pg_dir));
> diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
> index c7c2d89..04a83a7 100644
> --- a/arch/x86/xen/smp.h
> +++ b/arch/x86/xen/smp.h
> @@ -8,4 +8,12 @@ extern void xen_send_IPI_allbutself(int vector);
>  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 flag);
> +#else
> +static inline void xen_pvh_early_cpu_init(int cpu, bool flag);
> +{
> +}
> +#endif
> +
>  #endif
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 485b695..718f330 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -47,6 +47,39 @@ ENTRY(startup_xen)
>  
>  	__FINIT
>  
> +#ifdef CONFIG_XEN_PVH
> +/*
> + * xen_pvh_early_cpu_init() - early PVH VCPU initialization
> + * @cpu: this cpu number (%rdi)
> + * @flag: boolean flag true to indicate this is a secondary vcpu coming up
> + *        on this entry point or the primary cpu coming back online.

Why do we do this? Why not just piggyback on the first parameter - the 'cpu'?

If it is zero it is boot CPU.

> + *
> + * 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
> +
> +	/* Gather features to see if NX implemented. */
> +	mov     $0x80000001, %eax
> +	cpuid
> +	mov     %edx, %esi
> +
> +	mov     $MSR_EFER, %ecx
> +	rdmsr
> +	bts     $_EFER_SCE, %eax
> +
> +	bt      $20, %esi
> +	jnc     1f      	/* No NX, skip setting it */
> +	bts     $_EFER_NX, %eax
> +1:	wrmsr
> +
> +	cmp     $0, %r11b
> +	jne     cpu_bringup_and_idle
> +	ret
> +
> +#endif /* CONFIG_XEN_PVH */
> +
>  .pushsection .text
>  	.balign PAGE_SIZE
>  ENTRY(hypercall_page)
> -- 
> 1.8.3.1
> 
> 
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