Re: [PATCH v8 7/7] arm64: kvm: handle SError Interrupt by categorization

[gengdongjiu]

Hi James,
   Sorry for my late response due to out of office recently.

2018-01-13 2:05 GMT+08:00 James Morse <james.morse@xxxxxxx>:
> Hi gengdongjiu,
>
> On 15/12/17 03:30, gengdongjiu wrote:
>> On 2017/12/7 14:37, gengdongjiu wrote:
>>>> We need to tackle (1) and (3) separately. For (3) we need some API that lets
>>>> Qemu _trigger_ an SError in the guest, with a specified ESR. But, we don't have
>>>> a way of migrating pending SError yet... which is where I got stuck last time I
>>>> was looking at this.
>>> I understand you most idea.
>>>
>>> But In the Qemu one signal type can only correspond to one behavior, can not correspond to two behaviors,
>>> otherwise Qemu will do not know how to do.
>>>
>>> For the Qemu, if it receives the SIGBUS_MCEERR_AR signal, it will populate the CPER
>>> records and inject a SEA to guest through KVM IOCTL "KVM_SET_ONE_REG"; if receives the SIGBUS_MCEERR_AO
>>> signal, it will record the CPER and trigger a IRQ to notify guest, as shown below:
>>>
>>> SIGBUS_MCEERR_AR trigger Synchronous External Abort.
>>> SIGBUS_MCEERR_AO trigger GPIO IRQ.
>>>
>>> For the SIGBUS_MCEERR_AO and SIGBUS_MCEERR_AR, we have already specify trigger method, which all
>>>
>>> not involve _trigger_ an SError.
>>>
>>> so there is no chance for Qemu to trigger the SError when gets the SIGBUS_MCEERR_A{O,R}.
>>
>> As I explained above:
>>
>> If Qemu received SIGBUS_MCEERR_AR, it will record CPER and trigger Synchronous External Abort;
>> If Qemu received SIGBUS_MCEERR_AO, it will record CPER and trigger GPIO IRQ;
>
>> So Qemu does not know when to _trigger_ an SError.
>
> There is no answer to this. How the CPU decides is specific to the CPU design.
> How Qemu decides is going to be specific to the machine it emulates.
>
> My understanding is there is some overlap for which RAS errors are reported as
> synchronous external abort, and which use SError. (Obviously the imprecise ones
> are all SError). Which one the CPU uses depends on how the CPU is designed.
>
> When you take an SIGBUS_MCEERR_AR from KVM, its because KVM can't complete a
> stage2 fault because the page is marked with PG_poisoned. These started out as a
> synchronous exception, but you could still report these with SError.

yes, I agree, it is policy choice.

>
> We don't have a way to signal user-space about imprecise exceptions, this isn't
> a KVM specific problem.
>
>
>> so here I "return a error" to Qemu if ghes_notify_sei() return failure in [1], if you opposed KVM "return error",
>> do you have a better idea about it? thanks
>
> If ghes_notify_sei() fails to claim the error, we should drop through to
> kernel-first-handling. We don't have that yet, just the stub that ignores errors
> where we can make progress.
>
> If neither firmware-first nor kernel-first claim a RAS error, we're in trouble.
> I'd like to panic() as we got a RAS notification but no description of the
> error. We can't do this until we have kernel-first support, hence that stub.
>
>
>> About the way of migrating pending SError, I think it is a separate case, because Qemu still does not know
>> how and when to trigger the SError.
>
> I agree, but I think we should fix this first before we add another user of this
> unmigratable hypervisor state.
>
> (I recall someone saying migration is needed for any new KVM/cpu features, but I
> can't find the thread)
>
>
>> [1]:
>> static int kvm_handle_guest_sei(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> {
>>         .......................
>> +       case ESR_ELx_AET_UER:   /* The error has not been propagated */
>> +               /*
>> +                * Userspace only handle the guest SError Interrupt(SEI) if the
>> +                * error has not been propagated
>> +                */
>> +               run->exit_reason = KVM_EXIT_EXCEPTION;
>> +               run->ex.exception = ESR_ELx_EC_SERROR;
>
> I'm against telling user space RAS errors ever happened, only the final
> user-visible error when the kernel can't fix it.

thanks for the explanation.
For the ESR_ELx_AET_UER, this exception is precise, closing the VM may
be better[1].
But if you think panic is better until we support kernel-first, it is
also OK to me.


+static int kvm_handle_guest_sei(struct kvm_vcpu *vcpu, struct kvm_run *run)
+{
+     unsigned int esr = kvm_vcpu_get_hsr(vcpu);
+     bool impdef_syndrome =  esr & ESR_ELx_ISV; /* aka IDS */
+     unsigned int aet = esr & ESR_ELx_AET;
+
+     /*
+     * This is not RAS SError
+     */
+     if (!cpus_have_const_cap(ARM64_HAS_RAS_EXTN)) {
+         kvm_inject_vabt(vcpu);
+         return 1;
+     }
+
+     /* For RAS the host kernel may handle this abort. */
+     if (!handle_guest_sei())
+         return 1;
+
+     /*
+     * In below two conditions, it will directly inject the
+     * virtual SError:
+     * 1. The Syndrome is IMPLEMENTATION DEFINED
+     * 2. It is Uncategorized SEI
+     */
+     if (impdef_syndrome ||
+          ((esr & ESR_ELx_FSC) != ESR_ELx_FSC_SERROR)) {
+         kvm_inject_vabt(vcpu);
+         return 1;
+ }
+
+ switch (aet) {
+     case ESR_ELx_AET_CE: /* corrected error */
+     case ESR_ELx_AET_UEO: /* restartable error, not yet consumed */
+         return 1; /* continue processing the guest exit */
+     case ESR_ELx_AET_UER: /* recoverable error */
+         /*
+         * the exception is precise, not been silently propagated
+         * and not been consumed by the CPU, temporarily shut down
+         * the VM to isolated the error, hope not touch it again.
+         */
+     run->exit_reason = KVM_EXIT_EXCEPTION;
+     return 0;
+     default:
+     /*
+     * Until now, the CPU supports RAS, SError interrupt is fatal
+     * and host does not successfully handle it.
+     */
+     panic("This Asynchronous SError interrupt is dangerous, panic");
+    }
+
+     return 0;
+}
+

>
> This is inventing something new for RAS errors not claimed by firmware-first.
> If we have kernel-first too, this will never happen. (unless your system is
> losing the error description).
In fact, if we have kernel-first, I think we still need to judge the
error type by ESR, right?
If the handle_guest_sei() , may be the system does not support firmware-first,
so we judge the ESR value,

>
>
> Your system has firmware-first, why isn't it claiming the notification?
> If its not finding CPER records written by firmware, check firmware and the UEFI
> memory map agree on the attributes to be used when read/writing that area.
>
>
>> +               run->ex.error_code = KVM_SEI_SEV_RECOVERABLE;
>> +               return 0;
>
>
> Thanks,
>
> James
>
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