[xen staging-4.14] x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

[patchbot@xxxxxxx]

commit 5170ac955ba8e72a42dd95ebac2cd22cf5e875a1
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jan 28 11:57:19 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Feb 4 16:29:40 2022 +0000

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 00f2992b6c7a9d4090443c1a85bf83224a87eeb9)
---
 xen/arch/x86/acpi/power.c           |  3 +++
 xen/arch/x86/setup.c                |  5 ++++-
 xen/arch/x86/smpboot.c              |  5 +++++
 xen/arch/x86/spec_ctrl.c            | 10 +++++++---
 xen/include/asm-x86/current.h       |  2 +-
 xen/include/asm-x86/spec_ctrl_asm.h |  4 ++++
 6 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/acpi/power.c b/xen/arch/x86/acpi/power.c
index b67f9425b6..c856e4df40 100644
--- a/xen/arch/x86/acpi/power.c
+++ b/xen/arch/x86/acpi/power.c
@@ -294,7 +294,10 @@ 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 14654ee4df..d5274f8c21 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1928,9 +1928,12 @@ 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 64503df8e1..7edce1a139 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -321,6 +321,8 @@ 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.
@@ -377,7 +379,10 @@ 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 84d5de8856..b41f85936e 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1222,6 +1222,9 @@ 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;
 
         /*
@@ -1230,15 +1233,16 @@ 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 4d8822f78c..f732175a4f 100644
--- 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 bf82528a12..9c0c7622c4 100644
--- 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
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.14