Re: [PATCH 03/11] KVM: arm64: Change hyp_panic()s dependency on tpidr_el2

Christoffer Dall
2017-06-08T10:34:38Z

On Thu, Jun 08, 2017 at 11:23:55AM +0100, James Morse wrote:
> Hi Christoffer,
>
> On 06/06/17 20:45, Christoffer Dall wrote:
> > On Mon, May 15, 2017 at 06:43:51PM +0100, James Morse wrote:
> >> KVM calls hyp_panic() when anything unexpected happens. This may occur
> >> while a guest owns the EL1 registers. KVM stashes the vcpu pointer in
> >> tpidr_el2, which it uses to find the host context in order to restore
> >> the host EL1 registers before parachuting into the host's panic().
> >>
> >> The host context is a struct kvm_cpu_context allocated in the per-cpu
> >> area, and mapped to hyp. Given the per-cpu offset for this CPU, this is
> >> easy to find. Change hyp_panic() to take a pointer to the
> >> struct kvm_cpu_context. Wrap these calls with an asm function that
> >> retrieves the struct kvm_cpu_context from the host's per-cpu area.
> >>
> >> Copy the per-cpu offset from the hosts tpidr_el1 into tpidr_el2 during
> >> kvm init. (Later patches will make this unnecessary for VHE hosts)
> >>
> >> We print out the vcpu pointer as part of the panic message. Add a back
> >> reference to the 'running vcpu' in the host cpu context to preserve this.
>
> >> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> >> index fce7cc507e0a..0f83e7e402ad 100644
> >> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> >> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> >> @@ -163,6 +163,13 @@ ENTRY(__hyp_do_panic)
> >> eret
> >> ENDPROC(__hyp_do_panic)
> >>
> >> +ENTRY(__hyp_panic)
> >> + ldr x0, =kvm_host_cpu_state
> >
> > Why is this guaranteed to give us a valid EL2 address?
>
> ... it doesn't. Its a host address from the constant pool which is used by
> hyp-panic as __host_ctxt:
> > void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt);
>
> and fixed up with:
> > host_ctxt = kern_hyp_va(__host_ctxt);
>

Ah, right, we only dereference the pointer inside the function, I missed
that.

> to give us the EL2 address before we access it.
> For VHE the last step is a nop as the host address can be accessed directly.
>
> I will add a comment here explaining this.
>
>

Thanks!
-Christoffer