[Xen-devel] [PATCH v2 2/5] x86/domain: Initialise vcpu debug registers correctly

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

In particular, initialising %dr6 with the value 0 is buggy, because on
hardware supporting Transnational Memory, it will cause the sticky RTM bit to
be asserted, even though a debug exception from a transaction hasn't actually
been observed.

Introduce arch_vcpu_regs_init() to set various architectural defaults, and
reuse this in the hvm_vcpu_reset_state() path.

Architecturally, %edx's init state contains the processors model information,
and 0xf looks to be a remnant of the old Intel processors.  We clearly have no
software which cares, seeing as it is wrong for the last decade's worth of
Intel hardware and for all other vendors, so lets use the value 0 for
simplicity.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/domain.c        | 14 ++++++++++++++
 xen/arch/x86/hvm/hvm.c       |  6 ++----
 xen/include/asm-x86/domain.h |  2 ++
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 9371efc..6f19fbf 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -323,6 +323,18 @@ void free_vcpu_struct(struct vcpu *v)
     free_xenheap_page(v);
 }
 
+/* Initialise various registers to their architectural INIT/RESET state. */
+void arch_vcpu_regs_init(struct vcpu *v)
+{
+    v->arch.user_regs = (typeof(v->arch.user_regs)){
+        .rflags = X86_EFLAGS_MBS,
+    };
+
+    memset(v->arch.debugreg, 0, sizeof(v->arch.debugreg));
+    v->arch.debugreg[6] = X86_DR6_DEFAULT;
+    v->arch.debugreg[7] = X86_DR7_DEFAULT;
+}
+
 int arch_vcpu_create(struct vcpu *v)
 {
     struct domain *d = v->domain;
@@ -342,6 +354,8 @@ int arch_vcpu_create(struct vcpu *v)
             return rc;
 
         vmce_init_vcpu(v);
+
+        arch_vcpu_regs_init(v);
     }
     else if ( (rc = xstate_alloc_save_area(v)) != 0 )
         return rc;
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 9c105ff..f2b22e8 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3850,11 +3850,9 @@ void hvm_vcpu_reset_state(struct vcpu *v, uint16_t cs, 
uint16_t ip)
     vcpu_setup_fpu(v, v->arch.xsave_area, NULL, FCW_RESET);
 
     v->arch.vgc_flags = VGCF_online;
-    memset(&v->arch.user_regs, 0, sizeof(v->arch.user_regs));
-    v->arch.user_regs.rflags = X86_EFLAGS_MBS;
-    v->arch.user_regs.rdx = 0x00000f00;
+
+    arch_vcpu_regs_init(v);
     v->arch.user_regs.rip = ip;
-    memset(&v->arch.debugreg, 0, sizeof(v->arch.debugreg));
 
     v->arch.hvm.guest_cr[0] = X86_CR0_ET;
     hvm_update_guest_cr(v, 0);
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index cb0721e..cdb43e4 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -671,6 +671,8 @@ static inline void free_vcpu_guest_context(struct 
vcpu_guest_context *vgc)
     vfree(vgc);
 }
 
+void arch_vcpu_regs_init(struct vcpu *v);
+
 struct vcpu_hvm_context;
 int arch_set_info_hvm_guest(struct vcpu *v, const struct vcpu_hvm_context 
*ctx);
 
-- 
2.1.4


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