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[Xen-changelog] [xen master] x86/vmx: Fix handing of MSR_DEBUGCTL on VMExit



commit 730dc8d2c9e1b6402e66973cf99a7c56bc78be4c
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu May 24 17:20:09 2018 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Jul 4 12:12:14 2018 +0100

    x86/vmx: Fix handing of MSR_DEBUGCTL on VMExit
    
    Currently, whenever the guest writes a nonzero value to MSR_DEBUGCTL, Xen
    updates a host MSR load list entry with the current hardware value of
    MSR_DEBUGCTL.
    
    On VMExit, hardware automatically resets MSR_DEBUGCTL to 0.  Later, when the
    guest writes to MSR_DEBUGCTL, the current value in hardware (0) is fed back
    into guest load list.  As a practical result, `ler` debugging gets lost on 
any
    PCPU which has ever scheduled an HVM vcpu, and the common case when `ler`
    debugging isn't active, guest actions result in an unnecessary load list 
entry
    repeating the MSR_DEBUGCTL reset.
    
    Restoration of Xen's debugging setting needs to happen from the very first
    vmexit.  Due to the automatic reset, Xen need take no action in the general
    case, and only needs to load a value when debugging is active.
    
    This could be fixed by using a host MSR load list entry set up during
    construct_vmcs().  However, a more efficient option is to use an alternative
    block in the VMExit path, keyed on whether hypervisor debugging has been
    enabled.
    
    In order to set this up, drop the per cpu ler_msr variable (as there is no
    point having it per cpu when it will be the same everywhere), and use a 
single
    read_mostly variable instead.  Split calc_ler_msr() out of 
percpu_traps_init()
    for clarity.
    
    Finally, clean up do_debug().  Reinstate LBR early to help catch cascade
    errors, which allows for the removal of the out label.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/entry.S      |  9 ++++++
 xen/arch/x86/hvm/vmx/vmx.c        |  3 +-
 xen/arch/x86/traps.c              | 64 ++++++++++++++++++---------------------
 xen/arch/x86/x86_64/traps.c       |  7 +++--
 xen/include/asm-x86/cpufeature.h  |  1 +
 xen/include/asm-x86/cpufeatures.h |  1 +
 xen/include/asm-x86/msr.h         |  2 +-
 7 files changed, 47 insertions(+), 40 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/entry.S b/xen/arch/x86/hvm/vmx/entry.S
index aa2f103895..afd552f2b9 100644
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -41,6 +41,15 @@ ENTRY(vmx_asm_vmexit_handler)
         SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, Clob: 
acd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
+        /* Hardware clears MSR_DEBUGCTL on VMExit.  Reinstate it if debugging 
Xen. */
+        .macro restore_lbr
+            mov $IA32_DEBUGCTLMSR_LBR, %eax
+            mov $MSR_IA32_DEBUGCTLMSR, %ecx
+            xor %edx, %edx
+            wrmsr
+        .endm
+        ALTERNATIVE "", restore_lbr, X86_FEATURE_XEN_LBR
+
         mov  %rsp,%rdi
         call vmx_vmexit_handler
 
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 8e63eb7813..e319bf10ba 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3119,8 +3119,7 @@ static int vmx_msr_write_intercept(unsigned int msr, 
uint64_t msr_content)
                     }
         }
 
-        if ( (rc < 0) ||
-             (msr_content && (vmx_add_host_load_msr(msr) < 0)) )
+        if ( rc < 0 )
             hvm_inject_hw_exception(TRAP_machine_check, X86_EVENT_NO_EC);
         else
             __vmwrite(GUEST_IA32_DEBUGCTL, msr_content);
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 9f045a2045..789d7ff8cd 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -96,8 +96,6 @@ string_param("nmi", opt_nmi);
 DEFINE_PER_CPU(uint64_t, efer);
 static DEFINE_PER_CPU(unsigned long, last_extable_addr);
 
-DEFINE_PER_CPU_READ_MOSTLY(u32, ler_msr);
-
 DEFINE_PER_CPU_READ_MOSTLY(struct desc_struct *, gdt_table);
 DEFINE_PER_CPU_READ_MOSTLY(struct desc_struct *, compat_gdt_table);
 
@@ -117,6 +115,9 @@ integer_param("debug_stack_lines", debug_stack_lines);
 static bool opt_ler;
 boolean_param("ler", opt_ler);
 
+/* LastExceptionFromIP on this hardware.  Zero if LER is not in use. */
+unsigned int __read_mostly ler_msr;
+
 #define stack_words_per_line 4
 #define ESP_BEFORE_EXCEPTION(regs) ((unsigned long *)regs->rsp)
 
@@ -1778,17 +1779,6 @@ void do_device_not_available(struct cpu_user_regs *regs)
     return;
 }
 
-static void ler_enable(void)
-{
-    u64 debugctl;
-
-    if ( !this_cpu(ler_msr) )
-        return;
-
-    rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
-    wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl | IA32_DEBUGCTLMSR_LBR);
-}
-
 void do_debug(struct cpu_user_regs *regs)
 {
     unsigned long dr6;
@@ -1821,6 +1811,10 @@ void do_debug(struct cpu_user_regs *regs)
      */
     write_debugreg(6, X86_DR6_DEFAULT);
 
+    /* #DB automatically disabled LBR.  Reinstate it if debugging Xen. */
+    if ( cpu_has_xen_lbr )
+        wrmsrl(MSR_IA32_DEBUGCTLMSR, IA32_DEBUGCTLMSR_LBR);
+
     if ( !guest_mode(regs) )
     {
         /*
@@ -1838,7 +1832,7 @@ void do_debug(struct cpu_user_regs *regs)
             {
                 if ( regs->rip == (unsigned long)sysenter_eflags_saved )
                     regs->eflags &= ~X86_EFLAGS_TF;
-                goto out;
+                return;
             }
             if ( !debugger_trap_fatal(TRAP_debug, regs) )
             {
@@ -1895,20 +1889,14 @@ void do_debug(struct cpu_user_regs *regs)
                 regs->cs, _p(regs->rip), _p(regs->rip),
                 regs->ss, _p(regs->rsp), dr6);
 
-        goto out;
+        return;
     }
 
     /* Save debug status register where guest OS can peek at it */
     v->arch.debugreg[6] |= (dr6 & ~X86_DR6_DEFAULT);
     v->arch.debugreg[6] &= (dr6 | ~X86_DR6_DEFAULT);
 
-    ler_enable();
     pv_inject_hw_exception(TRAP_debug, X86_EVENT_NO_EC);
-    return;
-
- out:
-    ler_enable();
-    return;
 }
 
 static void __init noinline __set_intr_gate(unsigned int n,
@@ -1952,38 +1940,46 @@ void load_TR(void)
         : "=m" (old_gdt) : "rm" (TSS_ENTRY << 3), "m" (tss_gdt) : "memory" );
 }
 
-void percpu_traps_init(void)
+static unsigned int calc_ler_msr(void)
 {
-    subarch_percpu_traps_init();
-
-    if ( !opt_ler )
-        return;
-
     switch ( boot_cpu_data.x86_vendor )
     {
     case X86_VENDOR_INTEL:
         switch ( boot_cpu_data.x86 )
         {
         case 6:
-            this_cpu(ler_msr) = MSR_IA32_LASTINTFROMIP;
-            break;
+            return MSR_IA32_LASTINTFROMIP;
+
         case 15:
-            this_cpu(ler_msr) = MSR_P4_LER_FROM_LIP;
-            break;
+            return MSR_P4_LER_FROM_LIP;
         }
         break;
+
     case X86_VENDOR_AMD:
         switch ( boot_cpu_data.x86 )
         {
         case 6:
         case 0xf ... 0x17:
-            this_cpu(ler_msr) = MSR_IA32_LASTINTFROMIP;
-            break;
+            return MSR_IA32_LASTINTFROMIP;
         }
         break;
     }
 
-    ler_enable();
+    return 0;
+}
+
+void percpu_traps_init(void)
+{
+    subarch_percpu_traps_init();
+
+    if ( !opt_ler )
+        return;
+
+    if ( !ler_msr && (ler_msr = calc_ler_msr()) )
+        setup_force_cpu_cap(X86_FEATURE_XEN_LBR);
+
+    if ( cpu_has_xen_lbr )
+        wrmsrl(MSR_IA32_DEBUGCTLMSR, IA32_DEBUGCTLMSR_LBR);
 }
 
 void __init init_idt_traps(void)
diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index f7f6928d70..b0401850ef 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -144,11 +144,12 @@ void show_registers(const struct cpu_user_regs *regs)
     printk("CPU:    %d\n", smp_processor_id());
     _show_registers(&fault_regs, fault_crs, context, v);
 
-    if ( this_cpu(ler_msr) && !guest_mode(regs) )
+    if ( ler_msr && !guest_mode(regs) )
     {
         u64 from, to;
-        rdmsrl(this_cpu(ler_msr), from);
-        rdmsrl(this_cpu(ler_msr) + 1, to);
+
+        rdmsrl(ler_msr, from);
+        rdmsrl(ler_msr + 1, to);
         printk("ler: %016lx -> %016lx\n", from, to);
     }
 }
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 2b1f03c70e..4bc6c91ecd 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -112,6 +112,7 @@
 #define cpu_has_aperfmperf      boot_cpu_has(X86_FEATURE_APERFMPERF)
 #define cpu_has_lfence_dispatch boot_cpu_has(X86_FEATURE_LFENCE_DISPATCH)
 #define cpu_has_no_xpti         boot_cpu_has(X86_FEATURE_NO_XPTI)
+#define cpu_has_xen_lbr         boot_cpu_has(X86_FEATURE_XEN_LBR)
 
 enum _cache_type {
     CACHE_TYPE_NULL = 0,
diff --git a/xen/include/asm-x86/cpufeatures.h 
b/xen/include/asm-x86/cpufeatures.h
index b90aa2d046..8e5cc53dde 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -32,3 +32,4 @@ XEN_CPUFEATURE(SC_RSB_PV,       (FSCAPINTS+0)*32+18) /* RSB 
overwrite needed for
 XEN_CPUFEATURE(SC_RSB_HVM,      (FSCAPINTS+0)*32+19) /* RSB overwrite needed 
for HVM */
 XEN_CPUFEATURE(NO_XPTI,         (FSCAPINTS+0)*32+20) /* XPTI mitigation not in 
use */
 XEN_CPUFEATURE(SC_MSR_IDLE,     (FSCAPINTS+0)*32+21) /* (SC_MSR_PV || 
SC_MSR_HVM) && default_xen_spec_ctrl */
+XEN_CPUFEATURE(XEN_LBR,         (FSCAPINTS+0)*32+22) /* Xen uses 
MSR_DEBUGCTL.LBR */
diff --git a/xen/include/asm-x86/msr.h b/xen/include/asm-x86/msr.h
index 68ee7ea17a..d4064f442b 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -241,7 +241,7 @@ static inline void write_efer(uint64_t val)
     wrmsrl(MSR_EFER, val);
 }
 
-DECLARE_PER_CPU(u32, ler_msr);
+extern unsigned int ler_msr;
 
 DECLARE_PER_CPU(uint32_t, tsc_aux);
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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