Re: [PATCH v4 20/21] KVM: arm64: Take any host SError before entering the guest

Christoffer Dall
2017-11-03T12:47:20Z

On Thu, Nov 02, 2017 at 12:18:20PM +0000, James Morse wrote:
> Hi Christoffer,
>
> On 01/11/17 04:55, Christoffer Dall wrote:
> > On Tue, Oct 31, 2017 at 11:43:42AM +0000, James Morse wrote:
> >> On 31/10/17 06:23, Christoffer Dall wrote:
> >>> On Thu, Oct 19, 2017 at 03:58:06PM +0100, James Morse wrote:
> >>>> On VHE systems KVM masks SError before switching the VBAR value. Any
> >>>> host RAS error that the CPU knew about before world-switch may become
> >>>> pending as an SError during world-switch, and only be taken once we enter
> >>>> the guest.
> >>>>
> >>>> Until KVM can take RAS SErrors during world switch, add an ESB to
> >>>> force any RAS errors to be synchronised and taken on the host before
> >>>> we enter world switch.
> >>>>
> >>>> RAS errors that become pending during world switch are still taken
> >>>> once we enter the guest.
> >>
> >>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >>>> index cf5d78ba14b5..5dc6f2877762 100644
> >>>> --- a/arch/arm64/include/asm/kvm_host.h
> >>>> +++ b/arch/arm64/include/asm/kvm_host.h
> >>>> @@ -392,6 +392,7 @@ static inline void __cpu_init_stage2(void)
> >>>>
> >>>> static inline void kvm_arm_vhe_guest_enter(void)
> >>>> {
> >>>> + esb();
> >>
> >>> I don't fully appreciate what the point of this is?
> >>>
> >>> As I understand it, our fundamental goal here is to try to distinguish
> >>> between errors happening on the host or in the guest.
> >>
> >> Not just host/guest, but also those we can and can't handle.
> >>
> >> KVM can't currently take an SError during world switch, so a RAS error that the
> >> CPU was hoping to defer may spread from the host into KVM's
> >> no-SError:world-switch code. If this happens it will (almost certainly) have to
> >> be re-classified as uncontainable.
> >>
> >> There is also a firmware-first angle here: NOTIFY_SEI can't be delivered if the
> >> normal world has SError masked, so any error that spreads past this point
> >> becomes a reboot-by-firmware instead of an OS notification and almost-helpful
> >> error message.
> >>
> >>
> >>> If that's correct, then why don't we do it at the last possible moment
> >>> when we still have a scratch register left, in the world switch code
> >>> itself, and in the case abort the guest entry and report back a "host
> >>> SError" return code.
> >>
> >> We have IESB to run the error-barrier as we enter the guest. This would make any
> >> host error pending as an SError, and we would exit the guest immediately. But if
> >> there was an RAS error during world switch, by this point its likely to be
> >> classified as uncontainable.
> >>
> >> This esb() is trying to keep this window of code as small as possible, to just
> >> errors that occur during world switch.
> >>
> >> With your vcpu load/save this window becomes a lot smaller, it may be possible
> >> to get a VHE-host's arch-code SError handler to take errors from EL2, in which
> >> case this barrier can disappear.
> >> (note to self: guest may still own the debug hardware)
> >>
> >
> > ok, thanks for your detailed explanation. I didn't consider that the
> > classification of a RAS error as containable vs. non-containable
> > depended on where we take the exception.
>
> Will makes the point over on patch 11 that until we have different handling for
> these different classifications of error, there isn't much point doing this now.
> (i.e. we treat an error generated here, or when we enter the guest in the same way).
>
> I was trying to keep my eye on what we need for kernel-first support, so we
> don't have to change the code twice, we just expand the error handling to do better.

I figured as much...

>
> I'll drop this patch for now, it will come back if/when we get kernel-first
> support for RAS.

Either way is fine from my point of view.
>
>
> What about firmware-first? Firmware can always take these errors when the normal
> world is running. Dropping the barrier means its up to the CPU when any error
> gets reported, if firmware has to use NOTIFY_SEI it will have to do a reboot if
> the error occurs during world-switch (as SError is masked). If an error spreads
> over this boundary, that's just tough-luck, the kernel would have panic'd anyway.
>
>

Does a non-secure esb() cause the error to be delivered to firmware on
the secure side if anything is pending?

I'm not sure I fully understand the interaction between issuing an
esb() in non-secure and firmware handling a RAS error; I thought there
would be none, and that this was only for kernel-first ?

Thanks,
-Christoffer