[PATCH 03/11] x86: Initialise debug registers correctly

[Andrew Cooper <andrew.cooper3@citrix.com>]

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 event from a transaction hasn't actually been
observed.

Introduce X86_DR7_DEFAULT to match the existing X86_DR6_DEFAULT, and use
correct defaults when resetting the debug registers in cpu_init().

For vcpus, %dr6/7 have never been initialised.  In practice, this means that
toolstack get/set operations see zeros until the vcpu has first touched its
debug registers (at which point hardware fixes up the reserved bits), and the
RTM corner case will persist beyond that point.

Introduce initialise_registers() to set register defaults (including eflags
while we are fixing this) and call it early in vcpu_initialise().  Make a
similar adjustment in hvm_vcpu_reset_state().

Finally, adjust the vcpu state initialising logic in libxc.  All 3 sites zero
memory before choosing the nonzero defaults, which propagates the RTM corner
case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
---
 tools/libxc/xc_dom_x86.c       | 12 ++++++++++++
 xen/arch/x86/cpu/common.c      | 12 ++++++++----
 xen/arch/x86/domain.c          | 10 ++++++++++
 xen/arch/x86/hvm/hvm.c         |  6 +++++-
 xen/include/asm-x86/debugreg.h |  2 ++
 5 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/tools/libxc/xc_dom_x86.c b/tools/libxc/xc_dom_x86.c
index e33a288..3ab918c 100644
--- a/tools/libxc/xc_dom_x86.c
+++ b/tools/libxc/xc_dom_x86.c
@@ -53,6 +53,9 @@
 #define X86_CR0_PE 0x01
 #define X86_CR0_ET 0x10
 
+#define X86_DR6_DEFAULT 0xffff0ff0u
+#define X86_DR7_DEFAULT 0x00000400u
+
 #define SPECIALPAGE_PAGING   0
 #define SPECIALPAGE_ACCESS   1
 #define SPECIALPAGE_SHARING  2
@@ -860,6 +863,9 @@ static int vcpu_x86_32(struct xc_dom_image *dom)
         dom->parms.virt_base + (dom->start_info_pfn) * PAGE_SIZE_X86;
     ctxt->user_regs.eflags = 1 << 9; /* Interrupt Enable */
 
+    ctxt->debugreg[6] = X86_DR6_DEFAULT;
+    ctxt->debugreg[7] = X86_DR7_DEFAULT;
+
     ctxt->flags = VGCF_in_kernel_X86_32 | VGCF_online_X86_32;
     if ( dom->parms.pae == XEN_PAE_EXTCR3 ||
          dom->parms.pae == XEN_PAE_BIMODAL )
@@ -907,6 +913,9 @@ static int vcpu_x86_64(struct xc_dom_image *dom)
         dom->parms.virt_base + (dom->start_info_pfn) * PAGE_SIZE_X86;
     ctxt->user_regs.rflags = 1 << 9; /* Interrupt Enable */
 
+    ctxt->debugreg[6] = X86_DR6_DEFAULT;
+    ctxt->debugreg[7] = X86_DR7_DEFAULT;
+
     ctxt->flags = VGCF_in_kernel_X86_64 | VGCF_online_X86_64;
     cr3_pfn = xc_dom_p2m(dom, dom->pgtables_seg.pfn);
     ctxt->ctrlreg[3] = xen_pfn_to_cr3_x86_64(cr3_pfn);
@@ -1011,6 +1020,9 @@ static int vcpu_hvm(struct xc_dom_image *dom)
     /* Set the IP. */
     bsp_ctx.cpu.rip = dom->parms.phys_entry;
 
+    bsp_ctx.cpu.dr6 = X86_DR6_DEFAULT;
+    bsp_ctx.cpu.dr7 = X86_DR7_DEFAULT;
+
     if ( dom->start_info_seg.pfn )
         bsp_ctx.cpu.rbx = dom->start_info_seg.pfn << PAGE_SHIFT;
 
diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 528aff1..0872466 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -3,6 +3,7 @@
 #include <xen/delay.h>
 #include <xen/smp.h>
 #include <asm/current.h>
+#include <asm/debugreg.h>
 #include <asm/processor.h>
 #include <asm/xstate.h>
 #include <asm/msr.h>
@@ -823,10 +824,13 @@ void cpu_init(void)
 	/* Ensure FPU gets initialised for each domain. */
 	stts();
 
-	/* Clear all 6 debug registers: */
-#define CD(register) asm volatile ( "mov %0,%%db" #register : : "r"(0UL) );
-	CD(0); CD(1); CD(2); CD(3); /* no db4 and db5 */; CD(6); CD(7);
-#undef CD
+	/* Reset debug registers: */
+	write_debugreg(0, 0);
+	write_debugreg(1, 0);
+	write_debugreg(2, 0);
+	write_debugreg(3, 0);
+	write_debugreg(6, X86_DR6_DEFAULT);
+	write_debugreg(7, X86_DR7_DEFAULT);
 }
 
 void cpu_uninit(unsigned int cpu)
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 0ca820a..7ae9789 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -322,6 +322,14 @@ void free_vcpu_struct(struct vcpu *v)
     free_xenheap_page(v);
 }
 
+static void initialise_registers(struct vcpu *v)
+{
+    v->arch.user_regs.eflags = X86_EFLAGS_MBS;
+
+    v->arch.debugreg[6] = X86_DR6_DEFAULT;
+    v->arch.debugreg[7] = X86_DR7_DEFAULT;
+}
+
 int vcpu_initialise(struct vcpu *v)
 {
     struct domain *d = v->domain;
@@ -341,6 +349,8 @@ int vcpu_initialise(struct vcpu *v)
             return rc;
 
         vmce_init_vcpu(v);
+
+        initialise_registers(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 c23983c..10415e6 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -40,6 +40,7 @@
 #include <asm/shadow.h>
 #include <asm/hap.h>
 #include <asm/current.h>
+#include <asm/debugreg.h>
 #include <asm/e820.h>
 #include <asm/io.h>
 #include <asm/regs.h>
@@ -3907,7 +3908,10 @@ void hvm_vcpu_reset_state(struct vcpu *v, uint16_t cs, uint16_t ip)
     v->arch.user_regs.rflags = X86_EFLAGS_MBS;
     v->arch.user_regs.rdx = 0x00000f00;
     v->arch.user_regs.rip = ip;
-    memset(&v->arch.debugreg, 0, sizeof(v->arch.debugreg));
+
+    memset(&v->arch.debugreg, 0, sizeof(v->arch.debugreg) - 16);
+    v->arch.debugreg[6] = X86_DR6_DEFAULT;
+    v->arch.debugreg[7] = X86_DR7_DEFAULT;
 
     v->arch.hvm_vcpu.guest_cr[0] = X86_CR0_ET;
     hvm_update_guest_cr(v, 0);
diff --git a/xen/include/asm-x86/debugreg.h b/xen/include/asm-x86/debugreg.h
index b3b10ea..8e6a656 100644
--- a/xen/include/asm-x86/debugreg.h
+++ b/xen/include/asm-x86/debugreg.h
@@ -70,6 +70,8 @@
 #define DR_RTM_ENABLE            (0x00000800ul) /* RTM debugging enable */
 #define DR_GENERAL_DETECT        (0x00002000ul) /* General detect enable */
 
+#define X86_DR7_DEFAULT 0x00000400ul    /* Default %dr7 value. */
+
 #define write_debugreg(reg, val) do {                       \
     unsigned long __val = val;                              \
     asm volatile ( "mov %0,%%db" #reg : : "r" (__val) );    \
-- 
2.1.4


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