Re: [PATCH 4/4] KVM: vmx: Allow the guest to run with dirty debug registers
From: Paolo Bonzini
Date: Thu Feb 27 2014 - 07:55:08 EST
Il 27/02/2014 12:25, Jan Kiszka ha scritto:
On 2014-02-26 16:49, Paolo Bonzini wrote:
When not running in guest-debug mode (i.e. the guest controls the debug
registers, having to take an exit for each DR access is a waste of time.
If the guest gets into a state where each context switch causes DR to be
saved and restored, this can take away as much as 40% of the execution
time from the guest.
If the guest is running with vcpu->arch.db == vcpu->arch.eff_db, we
can let it write freely to the debug registers and reload them on the
next exit. We still need to exit on the first access, so that the
KVM_DEBUGREG_WONT_EXIT flag is set in switch_db_regs; after that, further
accesses to the debug registers will not cause a vmexit.
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 6e57e1434cf3..71c57ec48d8f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2851,7 +2851,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
vmx_capability.ept, vmx_capability.vpid);
}
- min = 0;
+ min = VM_EXIT_SAVE_DEBUG_CONTROLS;
#ifdef CONFIG_X86_64
min |= VM_EXIT_HOST_ADDR_SPACE_SIZE;
#endif
@@ -5121,6 +5121,22 @@ static int handle_dr(struct kvm_vcpu *vcpu)
}
}
+ if (vcpu->guest_debug == 0) {
+ u32 cpu_based_vm_exec_control;
+
+ cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
+ cpu_based_vm_exec_control &= ~CPU_BASED_MOV_DR_EXITING;
+ vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
+
+ /*
+ * No more DR vmexits; force a reload of the debug registers
+ * and reenter on this instruction. The next vmexit will
+ * retrieve the full state of the debug registers.
+ */
+ vcpu->arch.switch_db_regs |= KVM_DEBUGREG_WONT_EXIT;
+ return 1;
+ }
+
exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
dr = exit_qualification & DEBUG_REG_ACCESS_NUM;
reg = DEBUG_REG_ACCESS_REG(exit_qualification);
@@ -5147,6 +5163,18 @@ static void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
{
}
+static u64 vmx_get_dr7(struct kvm_vcpu *vcpu)
+{
+ /* DRs are being synced back to vcpu->arch, exit on DR access. */
+ u32 cpu_based_vm_exec_control;
+
+ cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
+ cpu_based_vm_exec_control |= CPU_BASED_MOV_DR_EXITING;
+ vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
+
+ return vmcs_readl(GUEST_DR7);
+}
The general idea looks ok (It passes x86/debug.flat unit test, right?).
Yes, of course.
But this side effect of get_dr7 seems a bit ugly to me. Also the
imbalanced updates of arch.switch_db_regs: KVM_DEBUGREG_WONT_EXIT is set
by the vendor code but cleared in a common x86 path.
I can certainly remove the difference in the updates of
KVM_DEBUGREG_WONT_EXIT. It made some sense when the constant was called
KVM_DEBUGREG_DIRTY but not now that I renamed it.
I don't like the side effect particularly, either, but I don't have any
better idea.
Paolo
Can't you make this more regular and explicit?
Jan
+
static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
{
vmcs_writel(GUEST_DR7, val);
@@ -8606,6 +8634,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
.set_gdt = vmx_set_gdt,
.get_dr6 = vmx_get_dr6,
.set_dr6 = vmx_set_dr6,
+ .get_dr7 = vmx_get_dr7,
.set_dr7 = vmx_set_dr7,
.cache_reg = vmx_cache_reg,
.get_rflags = vmx_get_rflags,
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/