x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/acpi/power.c b/xen/arch/x86/acpi/power.c
index 0837a3ead4dbdc7bab62caf83fd57a855e5d46fa..bac9c16389549b92d028af02a4ed88fe89ec8929 100644 (file)
--- 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_spec_ctrl);
+        ci->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/include/asm/current.h b/xen/arch/x86/include/asm/current.h
index cfbedc31983f8fa7805d22d62169b0ce0ca83b5a..dc0edd9ed07d136443ecd03dfe3a9f3066e35df4 100644 (file)
--- 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 bf82528a12ae521636c0979b6e768bc8ff6cca40..9c0c7622c41f4a399092b29bd3d736c2a37de3fa 100644 (file)
--- 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 e716005145d3aa7c4934c2e9aa8cd1ad7ec05cff..115f8f65173400f725e736354c4b4ed79b1cc19e 100644 (file)
--- 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 08c0f2d9df044207c4f3d137c3607ead6a0e00a6..1cfdf96207d4f3146c3de5a57b335b470ac9f338 100644 (file)
--- 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 2072daf66245bd15dac9ce413cffc3b3c04613f5..b2fd86ebe587a954ff58f6bb6c3d3281c89c7ce4 100644 (file)
--- 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) )