[PATCH v2 6/8] x86/hvm: Add comments about #DB exception behavior to {svm,vmx}_inject_event()

[Jinoh Kang <jinoh.kang.kr@xxxxxxxxx>]

From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Extracted comments only, and then s/from emulation/from monitor/;
originally "x86/hvm: RFC - PROBABLY BROKEN - Defer all debugging/monitor
actions to {svm,vmx}_inject_event()"

Signed-off-by: Jinoh Kang <jinoh.kang.kr@xxxxxxxxx>
---
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>

v1 -> v2: new patch
---
 xen/arch/x86/hvm/svm/svm.c | 9 +++++++++
 xen/arch/x86/hvm/vmx/vmx.c | 7 +++++++
 2 files changed, 16 insertions(+)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 6f3e6b3512..7bb572e72b 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1328,6 +1328,15 @@ static void cf_check svm_inject_event(const struct 
x86_event *event)
     switch ( _event.vector | -(_event.type == X86_EVENTTYPE_SW_INTERRUPT) )
     {
     case X86_EXC_DB:
+        /*
+         * On AMD hardware, a #DB exception:
+         *  1) Merges new status bits into %dr6
+         *  2) Clears %dr7.gd and MSR_DEBUGCTL.{LBR,BTF}
+         *
+         * Item 1 is done by hardware before a #DB intercepted vmexit, but we
+         * may end up here from monitor so have to repeat it ourselves.
+         * Item 2 is done by hardware when injecting a #DB exception.
+         */
         __restore_debug_registers(vmcb, curr);
         vmcb_set_dr6(vmcb, vmcb_get_dr6(vmcb) | _event.pending_dbg);
 
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 4e20fca43e..b35278992a 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2022,6 +2022,13 @@ static void cf_check vmx_inject_event(const struct 
x86_event *event)
     switch ( _event.vector | -(_event.type == X86_EVENTTYPE_SW_INTERRUPT) )
     {
     case X86_EXC_DB:
+        /*
+         * On Intel hardware, a #DB exception:
+         *  1) Merges new status bits into %dr6
+         *  2) Clears %dr7.gd and MSR_DEBUGCTL.LBR
+         *
+         * All actions are left up to the hypervisor to perform.
+         */
         __restore_debug_registers(curr);
         write_debugreg(6, read_debugreg(6) | event->pending_dbg);
 
-- 
2.41.0