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>
(cherry picked from commit 00f2992b6c7a9d4090443c1a85bf83224a87eeb9)

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/setup.c b/xen/arch/x86/setup.c
index da47cdea14a1b22fdbf0f409251aa0f98d6ee3f9..369691dd130ed5b6a2e5bbbb8da6f9b9ee611a34 100644 (file)
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1944,9 +1944,12 @@ 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 329cfdb6c9f65d7e3ef6e0488e7deb73ba5d48e5..ee3e86cc78b6c6a9c1b1ca4c5169582c60445954 100644 (file)
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -322,6 +322,8 @@ 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.
@@ -378,7 +380,10 @@ 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) )

diff --git a/xen/include/asm-x86/current.h b/xen/include/asm-x86/current.h
index a74ad4bc4c449692d93d9a74df6945ab7f9698b3..8ea4aecc5ef628815f16ffdd8fafe095f100f253 100644 (file)
--- a/xen/include/asm-x86/current.h
+++ b/xen/include/asm-x86/current.h
@@ -56,6 +56,7 @@ struct cpu_info {
     /* See asm-x86/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/include/asm-x86/spec_ctrl_asm.h b/xen/include/asm-x86/spec_ctrl_asm.h
index bf82528a12ae521636c0979b6e768bc8ff6cca40..9c0c7622c41f4a399092b29bd3d736c2a37de3fa 100644 (file)
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/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