On 27/01/2020 15:33, Valentin Schneider wrote:

On 18/12/2019 18:26, Ionela Voinescu wrote:

quoted

diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
index 6e5d839f42b5..dd20fb185d56 100644
--- a/arch/arm64/include/asm/kvm_arm.h
+++ b/arch/arm64/include/asm/kvm_arm.h

@@ -266,10 +266,11 @@
  #define CPTR_EL2_TFP_SHIFT 10
  
  /* Hyp Coprocessor Trap Register */
-#define CPTR_EL2_TCPAC	(1 << 31)
-#define CPTR_EL2_TTA	(1 << 20)
-#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
  #define CPTR_EL2_TZ	(1 << 8)
+#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
+#define CPTR_EL2_TTA	(1 << 20)
+#define CPTR_EL2_TAM	(1 << 30)
+#define CPTR_EL2_TCPAC	(1 << 31)

Nit: why the #define movement? Couldn't that just be added beneath
CPTR_EL2_TCPAC?

quoted

  #define CPTR_EL2_RES1	0x000032ff /* known RES1 bits in CPTR_EL2 */
  #define CPTR_EL2_DEFAULT	CPTR_EL2_RES1
  

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 72fbbd86eb5e..0bca87a2621f 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c

@@ -90,6 +90,17 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
  	val = read_sysreg(cpacr_el1);
  	val |= CPACR_EL1_TTA;
  	val &= ~CPACR_EL1_ZEN;
+
+	/*
+	 * With VHE enabled, we have HCR_EL2.{E2H,TGE} = {1,1}. Note that in
+	 * this case CPACR_EL1 has the same bit layout as CPTR_EL2, and
+	 * CPACR_EL1 accessing instructions are redefined to access CPTR_EL2.
+	 * Therefore use CPTR_EL2.TAM bit reference to activate AMU register
+	 * traps.
+	 */
+
+	val |= CPTR_EL2_TAM;
+

Hmm so this is a bit confusing for me, I've rewritten that part of the
email too many times (didn't help that I'm far from being a virt guru).
Rectifications are most welcome.


First, AFAICT we *don't* have HCR_EL2.TGE set anymore at this point, it's
cleared just a bit earlier in __activate_traps().


Then, your comment suggests that when we're running this code, CPACR_EL1
accesses are rerouted to CPTR_EL2. Annoyingly this isn't mentioned in
the doc of CPACR_EL1, but D5.6.3 does say

"""
When ARMv8.1-VHE is implemented, and HCR_EL2.E2H is set to 1, when executing
at EL2, some EL1 System register access instructions are redefined to access
the equivalent EL2 register.
"""

And CPACR_EL1 is part of these, so far so good. Now, the thing is
the doc for CPACR_EL1 *doesn't* mention any TAM bit - but CPTR_EL2 does.
I believe what *do* want here is to set CPTR_EL2.TAM (which IIUC we end
up doing via the rerouting).

So, providing I didn't get completely lost on the way, I have to ask:
why do we use CPACR_EL1 here? Couldn't we use CPTR_EL2 directly?

Part of the reason is, CPTR_EL2 has different layout depending on
whether HCR_EL2.E2H == 1. e.g, CPTR_EL2.TTA move from Bit[28] to Bit[20].

So, to keep it simple, CPTR_EL2 is used for non-VHE code with the shifts
as defined by the "CPTR_EL2 when E2H=0"

if E2H == 1, CPTR_EL2 takes the layout of CPACR_EL1 and "overrides" some
of the RES0 bits in CPACR_EL1 with EL2 controls (e.g: TAM, TCPAC).
Thus we use CPACR_EL1 to keep the "shifts" non-conflicting (e.g, ZEN)
and is the right thing to do.

It is a bit confusing, but we are doing the right thing. May be we could 
improve the comment like :

	/*
	 * With VHE (HCR.E2H == 1), CPTR_EL2 has the same layout as
	 * CPACR_EL1, except for some missing controls, such as TAM.
	 * And accesses to CPACR_EL1 are routed to CPTR_EL2.
	 * Also CPTR_EL2.TAM has the same position with or without
	 * HCR.E2H == 1. Therefore, use CPTR_EL2.TAM here for
	 * trapping the AMU accesses.
	 */

Suzuki

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