[Xen-devel] [PATCH 07/10] x86/spec_ctrl: Explicitly set Xen's default MSR_SPEC_CTRL value

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

With the impending ability to disable MSR_SPEC_CTRL handling on a
per-guest-type basis, the first exit-from-guest may not have the side effect
of loading Xen's choice of value.  Explicitly set Xen's default during the BSP
and AP boot paths.

For the BSP however, delay setting a non-zero MSR_SPEC_CTRL default until
after dom0 has been constructed when safe to do so.  Oracle report that this
speeds up boots of some hardware by 50s.

Reported-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/x86/setup.c            |  7 +++++++
 xen/arch/x86/smpboot.c          |  8 ++++++++
 xen/arch/x86/spec_ctrl.c        | 28 ++++++++++++++++++++++++++++
 xen/include/asm-x86/spec_ctrl.h |  2 ++
 4 files changed, 45 insertions(+)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 164c42c..a3172ca 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1743,6 +1743,13 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     setup_io_bitmap(dom0);
 
+    if ( bsp_delay_spec_ctrl )
+    {
+        get_cpu_info()->spec_ctrl_flags &= ~SCF_use_shadow;
+        barrier();
+        wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+    }
+
     /* Jump to the 1:1 virtual mappings of cpu0_stack. */
     asm volatile ("mov %[stk], %%rsp; jmp %c[fn]" ::
                   [stk] "g" (__va(__pa(get_stack_bottom()))),
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 86fa410..fd9050e 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -358,6 +358,14 @@ void start_secondary(void *unused)
     else
         microcode_resume_cpu(cpu);
 
+    /*
+     * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard
+     * any firmware settings.  Note: MSR_SPEC_CTRL may only become available
+     * after loading microcode.
+     */
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+        wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+
     if ( xen_guest )
         hypervisor_ap_setup();
 
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 0404962..de8b35f 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -38,6 +38,8 @@ static int8_t __initdata opt_ibrs = -1;
 static bool __initdata opt_rsb_pv = true;
 static bool __initdata opt_rsb_hvm = true;
 bool __read_mostly opt_ibpb = true;
+
+bool __initdata bsp_delay_spec_ctrl;
 uint8_t __read_mostly default_xen_spec_ctrl;
 uint8_t __read_mostly default_spec_ctrl_flags;
 
@@ -417,6 +419,32 @@ void __init init_speculation_mitigations(void)
         setup_clear_cpu_cap(X86_FEATURE_NO_XPTI);
 
     print_details(thunk, caps);
+
+    /*
+     * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard
+     * any firmware settings.  For performance reasons on native hardware, we
+     * delay applying non-zero settings until after dom0 has been constructed.
+     */
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    {
+        bsp_delay_spec_ctrl = !cpu_has_hypervisor && default_xen_spec_ctrl;
+
+        /*
+         * If delaying MSR_SPEC_CTRL setup, use the same mechanism as
+         * spec_ctrl_enter_idle(), by using a shadow value of zero.
+         */
+        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);
+    }
 }
 
 static void __init __maybe_unused build_assertions(void)
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 9880e19..bb4e7b2 100644
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -27,6 +27,8 @@
 void init_speculation_mitigations(void);
 
 extern bool opt_ibpb;
+
+extern bool bsp_delay_spec_ctrl;
 extern uint8_t default_xen_spec_ctrl;
 extern uint8_t default_spec_ctrl_flags;
 
-- 
2.1.4


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