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

James Morse
2017-10-31T11:43:56Z

Hi Christoffer,

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)

Thanks,

James

> If the answer to that question is, that since we will always have some
> instruction window before entering the guest and things will never be
> precise anyway, so we do it here where it's more convenient, then my
> counter-question would be why we do it at all then? If we're not
> precise anyway, then why not simply take our chances and hope that the
> hardware delivers the SError before we mask them, and if not, tough
> luck?