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Re: [Xen-devel] xen/arm: Software Step ARMv8 - PC stuck on instruction

On 03/08/17 11:46, James Morse wrote:
Hi Florian,

On 03/08/17 11:16, Florian Jakobsmeier wrote:
This is the current version:

I'm not familiar with Xen, so forgive my annotations:

asmlinkage void leave_hypervisor_tail(void)
+    /*This methode will be called after the 'guest_entry' macro in
/arch/arm64/entry.S set guest registers
+    Check single_step_enabled flag in domain struct here and set needed
+    */
+    struct vcpu *v = current;
+    if ( unlikely(v->domain->arch.monitor.singlestep_enabled ) )
+    {
+        if(!(guest_cpu_user_regs()->cpsr & 0b1000))

This tests SPSR.M[4], which is set for exits from AARCH32.

+        {

Sets MDSCR.EL1.SS to enable the state machine.


Sets TDE to trap debug exceptions to EL2 from lower exception levels. I'm
surprised this isn't always set for Xen. Do you allow guests to use the debug
features for their own purposes?

No. We already trap debug exceptions to EL2 by default and MDCR_EL2 should already be configured correctly for that.

+            guest_cpu_user_regs()->cpsr = guest_cpu_user_regs()->cpsr |

Sets SPSR.SS to suppress the step exception in the guest until its executed an

+            WRITE_SYSREG( READ_SYSREG(DAIF) & ~0x200, DAIF);

Here I'm confused. This looks like you are clearing PSTATE.D from the DAIF
register for EL2. This means debug exceptions are unmasked for exceptions from
Xen at EL2.

If you ERET with PSTATE.D clear the SPSR.SS bit will be discarded instead. I
thin this is your bug. The rules in 'D2.12.4 Entering the active-not-pending
state' require 'Debug exceptions are disabled from the current Exception level'.

You've set MDSCR_EL1.SS, if you also have MDSCR_EL1.KDE set you will start
single-stepping Xen once the register writes take effect, (which may be before
or after this:)

+            isb();

What are you synchronising here? (Unless you want to single-step Xen I suspect
you don't need this at all.)

Xen is been over cautious with isb at the moment :). I think this one is not necessary because you will affect EL1/EL0 context and it will be synchronized on the eret.

Julien Grall

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