[PATCHv4 0/2] x86/xen: PVH fixes

[PATCHv4 0/2] x86/xen: PVH fixes

[David Vrabel]

Two patches for PVH Linux guests.

This has been tested on an Intel processor (but not AMD).

Changes in v4 (David):
- cpu == 0 => boot CPU
- Reduce #ifdefs.
- Add patch for XEN_PVH docs.

Changes in v3 (David):
- Use common xen_pvh_cpu_early_init() function for boot and secondary
  VCPUs.

Changes in v2: (Mukesh):
- Use assembly macro to unify code for boot and secondary VCPUs.

[PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[David Vrabel]

From: Mukesh Rathor <mukesh.rathor@oracle.com>

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@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 arch/x86/xen/enlighten.c |    2 ++
 arch/x86/xen/smp.c       |   28 +++++++++++++++++-----------
 arch/x86/xen/smp.h       |    8 ++++++++
 arch/x86/xen/xen-head.S  |   30 ++++++++++++++++++++++++++++++
 4 files changed, 57 insertions(+), 11 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index c0cb11f..134d6a0 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1508,6 +1508,8 @@ static void __init xen_pvh_early_guest_init(void)
 		return;
 
 	xen_have_vector_callback = 1;
+
+	xen_pvh_cpu_early_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 7005974..1b668dc 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;
 
@@ -99,8 +100,12 @@ 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.
+ */
+asmlinkage __visible void cpu_bringup_and_idle(int cpu)
 {
 #ifdef CONFIG_X86_64
 	if (xen_feature(XENFEAT_auto_translated_physmap) &&
@@ -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 +417,17 @@ 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.
 		 */
+		ctxt->user_regs.eip = (unsigned long)xen_pvh_cpu_early_init;
 		ctxt->user_regs.rdi = cpu;
+	}
 #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..d5dbc65 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_cpu_early_init(int cpu);
+#else
+static inline void xen_pvh_cpu_early_init(int cpu)
+{
+}
+#endif
+
 #endif
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 485b695..62b3f10 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -47,6 +47,36 @@ ENTRY(startup_xen)
 
 	__FINIT
 
+#ifdef CONFIG_XEN_PVH
+
+/**
+ * xen_pvh_cpu_early_init() - early PVH VCPU initialization
+ * @cpu: this cpu number (%rdi)
+ *
+ * Note: This is called as a function on the boot CPU and as the secondary
+ * CPU entry point.
+ */
+ENTRY(xen_pvh_cpu_early_init)
+	/* 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,%rdi /* non-zero => secondary cpu */
+	jne     cpu_bringup_and_idle
+	ret
+
+#endif /* CONFIG_XEN_PVH */
+
 .pushsection .text
 	.balign PAGE_SIZE
 ENTRY(hypercall_page)
-- 
1.7.10.4

[PATCH 2/2] x86/xen: document CONFIG_XEN_PVH option

[David Vrabel]

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 arch/x86/xen/Kconfig |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
index e88fda8..77b7938 100644
--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -50,3 +50,11 @@ config XEN_PVH
 	bool "Support for running as a PVH guest"
 	depends on X86_64 && XEN && XEN_PVHVM
 	def_bool n
+	help
+	  PVH is an experimental guest mode that is a cross between
+	  full PV and HVM. The hypervisor ABI for PVH guests is not
+	  yet stable and this kernel may not boot in PVH mode on later
+	  Xen hypervisors.
+
+	  Do not enable this option unless you are a developer working
+	  on PVH guest support.
-- 
1.7.10.4

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[Jan Beulich]

>>> On 05.09.14 at 16:11, <david.vrabel@citrix.com> wrote:
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>  
>  	__FINIT
>  
> +#ifdef CONFIG_XEN_PVH
> +
> +/**
> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
> + * @cpu: this cpu number (%rdi)
> + *
> + * Note: This is called as a function on the boot CPU and as the secondary
> + * CPU entry point.
> + */
> +ENTRY(xen_pvh_cpu_early_init)
> +	/* 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,%rdi /* non-zero => secondary cpu */

Isn't Linux is specifically moving away from this assumption?

> +	jne     cpu_bringup_and_idle
> +	ret
> +
> +#endif /* CONFIG_XEN_PVH */

Also, does all this really need to be done in assembly?

Jan

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[David Vrabel]

On 05/09/14 15:34, Jan Beulich wrote:
>>>> On 05.09.14 at 16:11, <david.vrabel@citrix.com> wrote:
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>>  
>>  	__FINIT
>>  
>> +#ifdef CONFIG_XEN_PVH
>> +
>> +/**
>> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
>> + * @cpu: this cpu number (%rdi)
>> + *
>> + * Note: This is called as a function on the boot CPU and as the secondary
>> + * CPU entry point.
>> + */
>> +ENTRY(xen_pvh_cpu_early_init)
>> +	/* 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,%rdi /* non-zero => secondary cpu */
> 
> Isn't Linux is specifically moving away from this assumption?

I don't think this is relevant.  Is Xen ever going boot with a non-zero
VCPU?

>> +	jne     cpu_bringup_and_idle
>> +	ret
>> +
>> +#endif /* CONFIG_XEN_PVH */
> 
> Also, does all this really need to be done in assembly?

There's no usable stack until EFER.NX is set.  I couldn't think of a way
to write this in C that would guarantee no stack use, so I didn't
suggest it.

Can you think of a way?

David

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[Boris Ostrovsky]

On 09/05/2014 10:11 AM, David Vrabel wrote:
> From: Mukesh Rathor <mukesh.rathor@oracle.com>
>
> 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@oracle.com>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>   arch/x86/xen/enlighten.c |    2 ++
>   arch/x86/xen/smp.c       |   28 +++++++++++++++++-----------
>   arch/x86/xen/smp.h       |    8 ++++++++
>   arch/x86/xen/xen-head.S  |   30 ++++++++++++++++++++++++++++++
>   4 files changed, 57 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index c0cb11f..134d6a0 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1508,6 +1508,8 @@ static void __init xen_pvh_early_guest_init(void)
>   		return;
>   
>   	xen_have_vector_callback = 1;
> +
> +	xen_pvh_cpu_early_init(0);

Nit: for consistency may be better to call this xen_pvh_early_cpu_init().


>   	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 7005974..1b668dc 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;
>   
> @@ -99,8 +100,12 @@ 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.
> + */
> +asmlinkage __visible void cpu_bringup_and_idle(int cpu)
>   {
>   #ifdef CONFIG_X86_64
>   	if (xen_feature(XENFEAT_auto_translated_physmap) &&
> @@ -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 +417,17 @@ 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

Since you are wrapping this in ifdef here we should probably also do the 
same for xen_pvh_early_guest_init() xen_cpu_secondary_cpu_init().

-boris

> +	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.
>   		 */
> +		ctxt->user_regs.eip = (unsigned long)xen_pvh_cpu_early_init;
>   		ctxt->user_regs.rdi = cpu;
> +	}
>   #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..d5dbc65 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_cpu_early_init(int cpu);
> +#else
> +static inline void xen_pvh_cpu_early_init(int cpu)
> +{
> +}
> +#endif
> +
>   #endif
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 485b695..62b3f10 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>   
>   	__FINIT
>   
> +#ifdef CONFIG_XEN_PVH
> +
> +/**
> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
> + * @cpu: this cpu number (%rdi)
> + *
> + * Note: This is called as a function on the boot CPU and as the secondary
> + * CPU entry point.
> + */
> +ENTRY(xen_pvh_cpu_early_init)
> +	/* 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,%rdi /* non-zero => secondary cpu */
> +	jne     cpu_bringup_and_idle
> +	ret
> +
> +#endif /* CONFIG_XEN_PVH */
> +
>   .pushsection .text
>   	.balign PAGE_SIZE
>   ENTRY(hypercall_page)

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[Jan Beulich]

>>> On 05.09.14 at 16:46, <david.vrabel@citrix.com> wrote:
> On 05/09/14 15:34, Jan Beulich wrote:
>>>>> On 05.09.14 at 16:11, <david.vrabel@citrix.com> wrote:
>>> --- a/arch/x86/xen/xen-head.S
>>> +++ b/arch/x86/xen/xen-head.S
>>> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>>>  
>>>  	__FINIT
>>>  
>>> +#ifdef CONFIG_XEN_PVH
>>> +
>>> +/**
>>> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
>>> + * @cpu: this cpu number (%rdi)
>>> + *
>>> + * Note: This is called as a function on the boot CPU and as the secondary
>>> + * CPU entry point.
>>> + */
>>> +ENTRY(xen_pvh_cpu_early_init)
>>> +	/* 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,%rdi /* non-zero => secondary cpu */
>> 
>> Isn't Linux is specifically moving away from this assumption?
> 
> I don't think this is relevant.  Is Xen ever going boot with a non-zero
> VCPU?

That's not the question here. The question is - will CPU 0 ever be
possible to be brought down and back up? And I hope the answer
to this isn't "No, never".

>>> +	jne     cpu_bringup_and_idle
>>> +	ret
>>> +
>>> +#endif /* CONFIG_XEN_PVH */
>> 
>> Also, does all this really need to be done in assembly?
> 
> There's no usable stack until EFER.NX is set.  I couldn't think of a way
> to write this in C that would guarantee no stack use, so I didn't
> suggest it.
> 
> Can you think of a way?

No stack use is a very convincing argument for using assembly? I
must have overlooked it if that was said somewhere in a comment.

Jan

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[David Vrabel]

On 05/09/14 17:04, Jan Beulich wrote:
>>>> On 05.09.14 at 16:46, <david.vrabel@citrix.com> wrote:
>> On 05/09/14 15:34, Jan Beulich wrote:
>>>>>> On 05.09.14 at 16:11, <david.vrabel@citrix.com> wrote:
>>>> --- a/arch/x86/xen/xen-head.S
>>>> +++ b/arch/x86/xen/xen-head.S
>>>> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>>>>  
>>>>  	__FINIT
>>>>  
>>>> +#ifdef CONFIG_XEN_PVH
>>>> +
>>>> +/**
>>>> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
>>>> + * @cpu: this cpu number (%rdi)
>>>> + *
>>>> + * Note: This is called as a function on the boot CPU and as the secondary
>>>> + * CPU entry point.
>>>> + */
>>>> +ENTRY(xen_pvh_cpu_early_init)
>>>> +	/* 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,%rdi /* non-zero => secondary cpu */
>>>
>>> Isn't Linux is specifically moving away from this assumption?
>>
>> I don't think this is relevant.  Is Xen ever going boot with a non-zero
>> VCPU?
> 
> That's not the question here. The question is - will CPU 0 ever be
> possible to be brought down and back up? And I hope the answer
> to this isn't "No, never".

Oh, that's a good point. I'll add the flag back in.

>>>> +	jne     cpu_bringup_and_idle
>>>> +	ret
>>>> +
>>>> +#endif /* CONFIG_XEN_PVH */
>>>
>>> Also, does all this really need to be done in assembly?
>>
>> There's no usable stack until EFER.NX is set.  I couldn't think of a way
>> to write this in C that would guarantee no stack use, so I didn't
>> suggest it.
>>
>> Can you think of a way?
> 
> No stack use is a very convincing argument for using assembly?

Is that question mark a typo?  If not, I'm not really sure how to answer.

> I must have overlooked it if that was said somewhere in a comment.

>From the commit message:

"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()."

And a comment:

+	 * 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.

If this isn't sufficiently clear, can you suggest some improved wording?

David

Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[Jan Beulich]

>>> On 05.09.14 at 18:21, <david.vrabel@citrix.com> wrote:
> On 05/09/14 17:04, Jan Beulich wrote:
>>>>> On 05.09.14 at 16:46, <david.vrabel@citrix.com> wrote:
>>> There's no usable stack until EFER.NX is set.  I couldn't think of a way
>>> to write this in C that would guarantee no stack use, so I didn't
>>> suggest it.
>>>
>>> Can you think of a way?
>> 
>> No stack use is a very convincing argument for using assembly?
> 
> Is that question mark a typo?  If not, I'm not really sure how to answer.

Yes, it is. I'm sorry for the confusion.

Jan