On Tue, Nov 07, 2017 at 03:09:19PM +0100, Andrew Jones wrote:

On Thu, Oct 12, 2017 at 12:41:13PM +0200, Christoffer Dall wrote:

quoted

There is no need to figure out inside the world-switch if we should
save/restore the debug registers or not, we can might as well do that in
the higher level debug setup code, making it easier to optimize down the
line.

Signed-off-by: Christoffer Dall <redacted>
---
 arch/arm64/kvm/debug.c        | 9 +++++++++
 arch/arm64/kvm/hyp/debug-sr.c | 6 ------
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index dbadfaf..62550de19 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c

@@ -193,6 +193,15 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 	if (trap_debug)
 		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
 
+	/*
+	 * If any of KDE, MDE or KVM_ARM64_DEBUG_DIRTY is set, perform
+	 * a full save/restore cycle.

The commit message implies testing KVM_ARM64_DEBUG_DIRTY, but it only
tests KDE and MDE.

quoted

+	 */
+	if ((vcpu_sys_reg(vcpu, MDSCR_EL1) & DBG_MDSCR_KDE) ||
+	    (vcpu_sys_reg(vcpu, MDSCR_EL1) & DBG_MDSCR_MDE))

nit: could also write as

 if (vcpu_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))

 
Another nit: two empty lines at the end of this chunk.

quoted

+		vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
+

It looks like there's only one flag for debug_flags - this dirty flag,
which I guess is also used to trigger trapping. So maybe this could be a
second flag of a "lazy state" field, as I suggested earlier?

quoted

+
 	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
 	trace_kvm_arm_set_dreg32("MDSCR_EL1", vcpu_sys_reg(vcpu, MDSCR_EL1));
 }

diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
index f5154ed..0fc0758 100644
--- a/arch/arm64/kvm/hyp/debug-sr.c
+++ b/arch/arm64/kvm/hyp/debug-sr.c

@@ -172,12 +172,6 @@ void __hyp_text __debug_restore_state(struct kvm_vcpu *vcpu,
 
 void __hyp_text __debug_cond_save_host_state(struct kvm_vcpu *vcpu)
 {
-	/* If any of KDE, MDE or KVM_ARM64_DEBUG_DIRTY is set, perform
-	 * a full save/restore cycle. */
-	if ((vcpu->arch.ctxt.sys_regs[MDSCR_EL1] & DBG_MDSCR_KDE) ||
-	    (vcpu->arch.ctxt.sys_regs[MDSCR_EL1] & DBG_MDSCR_MDE))
-		vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
-
 	__debug_save_state(vcpu, &vcpu->arch.host_debug_state.regs,
 			   kern_hyp_va(vcpu->arch.host_cpu_context));
 	__debug_save_spe()(&vcpu->arch.host_debug_state.pmscr_el1);

-- 
2.9.0

Also, while grepping, I noticed __debug_cond_restore_host_state() unsets
KVM_ARM64_DEBUG_DIRTY on that condition that it's set. A micro
optimization could be to do it unconditionally, removing the branch.

Thanks,
drew