[PATCH 10/37] KVM: arm64: Slightly improve debug save/restore functions

Hi Julien,

On Tue, Nov 14, 2017 at 04:42:13PM +0000, Julien Thierry wrote:

On 12/10/17 11:41, Christoffer Dall wrote:

quoted

The debug save/restore functions can be improved by using the has_vhe()
static key instead of the instruction alternative.  Using the static key
uses the same paradigm as we're going to use elsewhere, it makes the
code more readable, and it generates slightly better code (no
stack setups and function calls unless necessary).

We also use a static key on the restore path, because it will be
marginally faster than loading a value from memory.

Finally, we don't have to conditionally clear the debug dirty flag if
it's set, we can just clear it.

Signed-off-by: Christoffer Dall <redacted>
---
 arch/arm64/kvm/hyp/debug-sr.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
index 0fc0758..a2291b6 100644
--- a/arch/arm64/kvm/hyp/debug-sr.c
+++ b/arch/arm64/kvm/hyp/debug-sr.c

@@ -75,11 +75,6 @@
 #define psb_csync()		asm volatile("hint #17")
-static void __hyp_text __debug_save_spe_vhe(u64 *pmscr_el1)
-{
-	/* The vcpu can run. but it can't hide. */
-}
-
 static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
 {
 	u64 reg;

@@ -109,10 +104,6 @@ static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
 	dsb(nsh);
 }
-static hyp_alternate_select(__debug_save_spe,
-			    __debug_save_spe_nvhe, __debug_save_spe_vhe,
-			    ARM64_HAS_VIRT_HOST_EXTN);
-
 static void __hyp_text __debug_restore_spe(u64 pmscr_el1)
 {
 	if (!pmscr_el1)

@@ -174,17 +165,22 @@ void __hyp_text __debug_cond_save_host_state(struct kvm_vcpu *vcpu)
 {
 	__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);
+
+	/* Non-VHE: Disable and flush SPE data generation
+	 * VHE: The vcpu can run. but it can't hide. */
+	if (!has_vhe())
+		__debug_save_spe_nvhe(&vcpu->arch.host_debug_state.pmscr_el1);
 }
 void __hyp_text __debug_cond_restore_host_state(struct kvm_vcpu *vcpu)
 {
-	__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);
+	if (!has_vhe())
+		__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);

For consistency, would it be worth naming that function
'__debug_restore_spe_nvhe' ?

Yes.

Also, looking at __debug_save_spe_nvhe, I'm not sure how we guarantee that
we might not end up using stale data during the restore_spe (though, if this
is an issue, it existed before this change).
The save function might exit without setting a value to saved pmscr_el1.

Basically I'm wondering if the following scenario (in non VHE) is possible
and/or whether it is problematic:

- save spe
- restore spe
- host starts using spi -> !(PMBLIMITR_EL1 & PMBLIMITR_EL1_E)

spi ?

- save spe -> returns early without setting pmscr_el1
- restore spe with old save instead of doing nothing

I think I see what you mean.  Basically you're asking if we need this:

diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
index 4112160..8ab3510 100644
--- a/arch/arm64/kvm/hyp/debug-sr.c
+++ b/arch/arm64/kvm/hyp/debug-sr.c

@@ -106,7 +106,7 @@ static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
 
 static void __hyp_text __debug_restore_spe_nvhe(u64 &pmscr_el1)
 {
-	if (!pmscr_el1)
+	if (*pmscr_el1 != 0)
 		return;
 
 	/* The host page table is installed, but not yet synchronised */

@@ -114,6 +114,7 @@ static void __hyp_text __debug_restore_spe_nvhe(u64 &pmscr_el1)
 
 	/* Re-enable data generation */
 	write_sysreg_s(pmscr_el1, PMSCR_EL1);
+	*pmscr_el1 = 0;
 }
 
 void __hyp_text __debug_save_state(struct kvm_vcpu *vcpu,

I think we do, and I think this is a separate fix.  Would you like to
write a patch and cc Will and Marc (original author and committer) to
fix this?  Probably worth a cc stable as well.

Thanks,
-Christoffer