[Xen-devel] [PATCH v2 3/5] tools/dombuilder: 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.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

The correct way to do this would be to get/modify/set the vcpu state, but it
turns out that is impossible for an HVM vcpu which hasn't yet had state set.
Fixing that is going to take some substantial untangling from implications in
the migration stream.
---
 tools/libxc/xc_dom_x86.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/libxc/xc_dom_x86.c b/tools/libxc/xc_dom_x86.c
index 77a4c6c..9e279d6 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 MTRR_TYPE_WRBACK     6
 #define MTRR_DEF_TYPE_ENABLE (1u << 11)
 
@@ -863,6 +866,9 @@ 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 )
@@ -910,6 +916,9 @@ 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);
@@ -1030,6 +1039,9 @@ 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;
 
-- 
2.1.4


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