[PATCH v2 5/9] x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

In some cases, writes to MSR_SPEC_CTRL do not have interesting side effects,
and we should implement lazy context switching like we do with other MSRs.

In the short term, this will be used by the SVM infrastructure, but I expect
to extend it to other contexts in due course.

Introduce cpu_info.last_spec_ctrl for the purpose, and cache writes made from
the boot/resume paths.  The value can't live in regular per-cpu data when it
is eventually used for PV guests when XPTI might be active.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

v2:
 * New
---
 xen/arch/x86/acpi/power.c                |  3 +++
 xen/arch/x86/include/asm/current.h       |  2 +-
 xen/arch/x86/include/asm/spec_ctrl_asm.h |  4 ++++
 xen/arch/x86/setup.c                     |  5 ++++-
 xen/arch/x86/smpboot.c                   |  5 +++++
 xen/arch/x86/spec_ctrl.c                 | 10 +++++++---
 6 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/acpi/power.c b/xen/arch/x86/acpi/power.c
index ea2bd8bbfe93..5f2ec74f744a 100644
--- a/xen/arch/x86/acpi/power.c
+++ b/xen/arch/x86/acpi/power.c
@@ -296,7 +296,10 @@ static int enter_state(u32 state)
     ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
 
     if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    {
         wrmsrl(MSR_SPEC_CTRL, default_xen_mcu_opt_ctrl);
+        ci->last_spec_ctrl = default_xen_mcu_opt_ctrl;
+    }
 
     if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
         wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
diff --git a/xen/arch/x86/include/asm/current.h 
b/xen/arch/x86/include/asm/current.h
index cfbedc31983f..dc0edd9ed07d 100644
--- a/xen/arch/x86/include/asm/current.h
+++ b/xen/arch/x86/include/asm/current.h
@@ -56,6 +56,7 @@ struct cpu_info {
     /* See asm/spec_ctrl_asm.h for usage. */
     unsigned int shadow_spec_ctrl;
     uint8_t      xen_spec_ctrl;
+    uint8_t      last_spec_ctrl;
     uint8_t      spec_ctrl_flags;
 
     /*
@@ -73,7 +74,6 @@ struct cpu_info {
      */
     bool         use_pv_cr3;
 
-    unsigned long __pad;
     /* get_stack_bottom() must be 16-byte aligned */
 };
 
diff --git a/xen/arch/x86/include/asm/spec_ctrl_asm.h 
b/xen/arch/x86/include/asm/spec_ctrl_asm.h
index bf82528a12ae..9c0c7622c41f 100644
--- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
+++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
@@ -67,6 +67,10 @@
  * steps 2 and 6 will restore the shadow value rather than leaving Xen's value
  * loaded and corrupting the value used in guest context.
  *
+ * Additionally, in some cases it is safe to skip writes to MSR_SPEC_CTRL when
+ * we don't require any of the side effects of an identical write.  Maintain a
+ * per-cpu last_spec_ctrl value for this purpose.
+ *
  * The following ASM fragments implement this algorithm.  See their local
  * comments for further details.
  *  - SPEC_CTRL_ENTRY_FROM_PV
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index e716005145d3..115f8f651734 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1930,9 +1930,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     if ( bsp_delay_spec_ctrl )
     {
-        get_cpu_info()->spec_ctrl_flags &= ~SCF_use_shadow;
+        struct cpu_info *info = get_cpu_info();
+
+        info->spec_ctrl_flags &= ~SCF_use_shadow;
         barrier();
         wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+        info->last_spec_ctrl = default_xen_spec_ctrl;
     }
 
     /* Jump to the 1:1 virtual mappings of cpu0_stack. */
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 08c0f2d9df04..1cfdf96207d4 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -323,6 +323,8 @@ static void set_cpu_sibling_map(unsigned int cpu)
 
 void start_secondary(void *unused)
 {
+    struct cpu_info *info = get_cpu_info();
+
     /*
      * Dont put anything before smp_callin(), SMP booting is so fragile that we
      * want to limit the things done here to the most necessary things.
@@ -379,7 +381,10 @@ void start_secondary(void *unused)
      * microcode.
      */
     if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    {
         wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+        info->last_spec_ctrl = default_xen_spec_ctrl;
+    }
     if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
         wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
 
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 2072daf66245..b2fd86ebe587 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1270,6 +1270,9 @@ void __init init_speculation_mitigations(void)
      */
     if ( has_spec_ctrl )
     {
+        struct cpu_info *info = get_cpu_info();
+        unsigned int val;
+
         bsp_delay_spec_ctrl = !cpu_has_hypervisor && default_xen_spec_ctrl;
 
         /*
@@ -1278,15 +1281,16 @@ void __init init_speculation_mitigations(void)
          */
         if ( bsp_delay_spec_ctrl )
         {
-            struct cpu_info *info = get_cpu_info();
-
             info->shadow_spec_ctrl = 0;
             barrier();
             info->spec_ctrl_flags |= SCF_use_shadow;
             barrier();
         }
 
-        wrmsrl(MSR_SPEC_CTRL, bsp_delay_spec_ctrl ? 0 : default_xen_spec_ctrl);
+        val = bsp_delay_spec_ctrl ? 0 : default_xen_spec_ctrl;
+
+        wrmsrl(MSR_SPEC_CTRL, val);
+        info->last_spec_ctrl = val;
     }
 
     if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
-- 
2.11.0