Re: [PATCH] KVM: arm64: stop propagating DAIF flags between kernel and VHE's world switch

Christoffer Dall
2017-08-30T19:02:39Z

On Wed, Aug 30, 2017 at 07:01:39PM +0100, James Morse wrote:
> Hi Christoffer,
>
> On 30/08/17 09:33, Christoffer Dall wrote:
> > On Tue, Aug 29, 2017 at 06:10:57PM +0100, James Morse wrote:
> >> On 24/08/17 16:23, Christoffer Dall wrote:
> >>> On Thu, Aug 10, 2017 at 12:30:21PM +0100, James Morse wrote:
> >>>> debug exceptions remain disabled due to the guest exit exception,
> >>>> (as does SError: today this is the only time SError is unmasked in the
> >>>> kernel). The flags stay in this state until we return to userspace.
> >>>>
> >>>> We have a __vhe_hyp_call() function that does the isb that we implicitly
> >>>> have on non-vhe systems, add the DAIF save/restore here, instead of in
> >>>> __sysreg_{save,restore}_host_state() which would require an extra isb()
> >>>> between the hosts VBAR_EL1 being restored and DAIF being restored.
> >>
> >>> This also means that anyone else calling kvm_call_hyp(), which we are
> >>> beginning to do more often, would also do this save/restore which
> >>> shouldn't really be necessary, doesn't it?
> >>
> >> I think it is, its done implicitly via SPSR_EL2 by the HVC/ERET on non-VHE.
> >> Does the HYP code on the other end of kvm_call_hyp() expect to be called with
> >> DAIF masked? What about the other way, if the HYP code modifies the DAIF flags
> >> should that spread back into the kernel?
> >
> > Well, for VHE I don't think this is any different than any other
> > function. There really is no concept of 'hyp code' --- or we should aim
> > for there not to be --- with VHE, so if some code expects interrupts
> > disabled or other changes to the DAIF flags, that code should really do
> > that for VHE.
>
> Aha, this is where I see the world differently.
> /me adjusts world-view,
>
> I will scrap this patch and re-do it along these lines.
>
>
> > The rationale being that in the long run we want to keep "jumping to
> > hyp" the oddball legacy case, where everything else is just the
> > kernel/hypervisor functionality.
>
> This makes sense.
>
>
> [ ... trims your excellent argument ... ]
>
>
> > Have we actually looked at the places where we call kvm_call_hyp() and
> > do they require a different setting of the DAIF flags from other kernel
> > code running at EL2 with VHE ?
>
> I can go through them, but I think its just world-switch as it modifies
> debug-registers, vbar_el1 and enters/exits the guest.
>
>
> > I understand that the behavior is currently different, but what I'm
> > asking is, instead of having to save and restore anything to the stack,
> > can you simply do:
> >
> > __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> > {
> > if (has_vhe())
> > asm("msr daifset, #0xf");
> >
> > ...
> > exit_code = __guest_enter(vcpu, host_ctxt);
> > ...
> >
> > if (has_vhe())
> > asm("msr daifclr, #0xd");
> > }
>
> Sure, this can be done as late as setting vbar_el1 for VHE, at which point the
> reason for masking these is clear. Before this point the hosts vectors will
> happily handle debug/fiq/serror.
>
> Your right KVM can 'just know' the values to set so the noise around
>
>
> > (not sure about the FIQ flag - does the kernel usually
>
> This is some of the stuff we need to clear up. Today we never expect SError or
> FIQ and should panic() if we get one. But these flags are largely ignored so
> when the CPU masks them on exception we leave them like that.
> After the SError rework process-context should have everything unmasked, today
> its just debug+irqs unmasked.
>
>
> Sorry if this email exchange has been frustrating, I didn't have your view of
> where all this should end up.
>

Not frustrating at all, I should have explained my rationale in my
initial reply.

Thanks,
-Christoffer