[Xen-devel] [PATCH v3 4/4] x86/asm: Rewrite sync_core() to use IRET-to-self

[Andy Lutomirski <luto@xxxxxxxxxx>]

Aside from being excessively slow, CPUID is problematic: Linux runs
on a handful of CPUs that don't have CPUID.  Use IRET-to-self
instead.  IRET-to-self works everywhere, so it makes testing easy.

For reference, On my laptop, IRET-to-self is ~110ns,
CPUID(eax=1, ecx=0) is ~83ns on native and very very slow under KVM,
and MOV-to-CR2 is ~42ns.

While we're at it: sync_core() serves a very specific purpose.
Document it.

Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/processor.h | 77 ++++++++++++++++++++++++++++------------
 1 file changed, 55 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 64fbc937d586..201a956e345f 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -590,33 +590,66 @@ static __always_inline void cpu_relax(void)
 
 #define cpu_relax_lowlatency() cpu_relax()
 
-/* Stop speculative execution and prefetching of modified code. */
+/*
+ * This function forces the icache and prefetched instruction stream to
+ * catch up with reality in two very specific cases:
+ *
+ *  a) Text was modified using one virtual address and is about to be executed
+ *     from the same physical page at a different virtual address.
+ *
+ *  b) Text was modified on a different CPU, may subsequently be
+ *     executed on this CPU, and you want to make sure the new version
+ *     gets executed.  This generally means you're calling this in a IPI.
+ *
+ * If you're calling this for a different reason, you're probably doing
+ * it wrong.
+ */
 static inline void sync_core(void)
 {
-       int tmp;
-
-#ifdef CONFIG_X86_32
        /*
-        * Do a CPUID if available, otherwise do a jump.  The jump
-        * can conveniently enough be the jump around CPUID.
+        * There are quite a few ways to do this.  IRET-to-self is nice
+        * because it works on every CPU, at any CPL (so it's compatible
+        * with paravirtualization), and it never exits to a hypervisor.
+        * The only down sides are that it's a bit slow (it seems to be
+        * a bit more than 2x slower than the fastest options) and that
+        * it unmasks NMIs.  The "push %cs" is needed because, in
+        * paravirtual environments, __KERNEL_CS may not be a valid CS
+        * value when we do IRET directly.
+        *
+        * In case NMI unmasking or performance every becomes a problem,
+        * the next best option appears to be MOV-to-CR2 and an
+        * unconditional jump.  That sequence also works on all CPUs,
+        * but it will fault at CPL3.
+        *
+        * CPUID is the conventional way, but it's nasty: it doesn't
+        * exist on some 486-like CPUs, and it usually exits to a
+        * hypervisor.
         */
-       asm volatile("cmpl %2,%1\n\t"
-                    "jl 1f\n\t"
-                    "cpuid\n"
-                    "1:"
-                    : "=a" (tmp)
-                    : "rm" (boot_cpu_data.cpuid_level), "ri" (0), "0" (1)
-                    : "ebx", "ecx", "edx", "memory");
+       register void *__sp asm(_ASM_SP);
+
+#ifdef CONFIG_X86_32
+       asm volatile (
+               "pushfl\n\t"
+               "pushl %%cs\n\t"
+               "pushl $1f\n\t"
+               "iret\n\t"
+               "1:"
+               : "+r" (__sp) : : "cc", "memory");
 #else
-       /*
-        * CPUID is a barrier to speculative execution.
-        * Prefetched instructions are automatically
-        * invalidated when modified.
-        */
-       asm volatile("cpuid"
-                    : "=a" (tmp)
-                    : "0" (1)
-                    : "ebx", "ecx", "edx", "memory");
+       unsigned long tmp;
+
+       asm volatile (
+               "movq %%ss, %0\n\t"
+               "pushq %0\n\t"
+               "pushq %%rsp\n\t"
+               "addq $8, (%%rsp)\n\t"
+               "pushfq\n\t"
+               "movq %%cs, %0\n\t"
+               "pushq %0\n\t"
+               "pushq $1f\n\t"
+               "iretq\n\t"
+               "1:"
+               : "=r" (tmp), "+r" (__sp) : : "cc", "memory");
 #endif
 }
 
-- 
2.9.3


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