[Xen-devel] [PATCH 34 of 36] x86_64: swapgs pvop with a user-stack can never be called

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

It's never safe to call a swapgs pvop when the user stack is current -
it must be inline replaced.  Rather than making a call, the
SWAPGS_UNSAFE_STACK pvop always just puts "swapgs" as a placeholder,
which must either be replaced inline or trap'n'emulated (somehow).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 include/asm-x86/irqflags.h |    2 +-
 include/asm-x86/paravirt.h |   10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/asm-x86/irqflags.h b/include/asm-x86/irqflags.h
--- a/include/asm-x86/irqflags.h
+++ b/include/asm-x86/irqflags.h
@@ -167,6 +167,7 @@
 #define INTERRUPT_RETURN_NMI_SAFE      NATIVE_INTERRUPT_RETURN_NMI_SAFE
 
 #ifdef CONFIG_X86_64
+#define SWAPGS_UNSAFE_STACK    swapgs
 #define INTERRUPT_RETURN       iretq
 #define USERGS_SYSRET64                                \
        swapgs;                                 \
@@ -241,7 +242,6 @@
  * Either way, this is a good way to document that we don't
  * have a reliable stack. x86_64 only.
  */
-#define SWAPGS_UNSAFE_STACK    swapgs
 #define ARCH_LOCKDEP_SYS_EXIT          call lockdep_sys_exit_thunk
 #define ARCH_LOCKDEP_SYS_EXIT_IRQ      \
        TRACE_IRQS_ON; \
diff --git a/include/asm-x86/paravirt.h b/include/asm-x86/paravirt.h
--- a/include/asm-x86/paravirt.h
+++ b/include/asm-x86/paravirt.h
@@ -1529,6 +1529,16 @@
 
 
 #else  /* !CONFIG_X86_32 */
+
+/*
+ * If swapgs is used while the userspace stack is still current,
+ * there's no way to call a pvop.  The PV replacement *must* be
+ * inlined, or the swapgs instruction must be trapped and emulated.
+ */
+#define SWAPGS_UNSAFE_STACK                                            \
+       PARA_SITE(PARA_PATCH(pv_cpu_ops, PV_CPU_swapgs), CLBR_NONE,     \
+                 swapgs)
+
 #define SWAPGS                                                         \
        PARA_SITE(PARA_PATCH(pv_cpu_ops, PV_CPU_swapgs), CLBR_NONE,     \
                  PV_SAVE_REGS;                                         \



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