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

James Morse
2017-11-02T12:18:26Z

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

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.

Sorry for the noise,

Thanks,

James