[RFC 1/3] x86: Add this_cpu_sp0() to read sp0 for the current cpu

[Andy Lutomirski]

We currently store references to the top of the kernel stack in
multiple places: kernel_stack (with an offset) and
init_tss.x86_tss.sp0 (no offset).  The latter is defined by hardware
and is a clean canonical way to find the top of the stack.  Add an
accessor so we can start using it.

This needs minor paravirt tweaks to ensure that On native, sp0
defines the top of the kernel stack.  On Xen and lguest, the
hypervisor tracks it, but we want to start reading sp0 in the
kernel.  Fixing this is simple: just update our local copy of sp0 as
well as the hypervisor's copy on task switches.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/include/asm/processor.h | 5 +++++
 arch/x86/lguest/boot.c           | 1 +
 arch/x86/xen/enlighten.c         | 1 +
 3 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index a092a0cce0b7..477b1ff4a6c9 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -564,6 +564,11 @@ static inline void native_swapgs(void)
 #endif
 }
 
+static inline unsigned long this_cpu_sp0(void)
+{
+	return this_cpu_read_stable(init_tss.x86_tss.sp0);
+}
+
 #ifdef CONFIG_PARAVIRT
 #include <asm/paravirt.h>
 #else
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index c1c1544b8485..77a609d1898d 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -1053,6 +1053,7 @@ static void lguest_load_sp0(struct tss_struct *tss,
 {
 	lazy_hcall3(LHCALL_SET_STACK, __KERNEL_DS | 0x1, thread->sp0,
 		   THREAD_SIZE / PAGE_SIZE);
+	tss->x86_tss.sp0 = sp0;
 }
 
 /* Let's just say, I wouldn't do debugging under a Guest. */
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 78a881b7fc41..d8195ca15346 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -912,6 +912,7 @@ static void xen_load_sp0(struct tss_struct *tss,
 	mcs = xen_mc_entry(0);
 	MULTI_stack_switch(mcs.mc, __KERNEL_DS, thread->sp0);
 	xen_mc_issue(PARAVIRT_LAZY_CPU);
+	tss->x86_tss.sp0 = thread->sp0;
 }
 
 static void xen_set_iopl_mask(unsigned mask)
-- 
2.1.0

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