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Subject: Re: [PATCH V12 10/10] arm/arm64: KVM: add guest SEA support
To: James Morse <james.morse@arm.com>
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From: "Baicar, Tyler" <tbaicar@codeaurora.org>
Message-ID: <9e2dee3d-afb3-72f6-a8f2-4fa090e87ee8@codeaurora.org>
Date: Fri, 10 Mar 2017 11:15:29 -0700
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Hello James,


On 3/8/2017 9:09 AM, James Morse wrote:
> On 07/03/17 17:58, Baicar, Tyler wrote:
>> On 3/7/2017 4:48 AM, James Morse wrote:
>>> On 06/03/17 20:45, Tyler Baicar wrote:
>>>> Currently external aborts are unsupported by the guest abort
>>>> handling. Add handling for SEAs so that the host kernel reports
>>>> SEAs which occur in the guest kernel.
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index a5265ed..f3608c9 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -1444,8 +1463,21 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu,
>>>> struct kvm_run *run)
>>>>          /* Check the stage-2 fault is trans. fault or write fault */
>>>>        fault_status = kvm_vcpu_trap_get_fault_type(vcpu);
>>>> -    if (fault_status != FSC_FAULT && fault_status != FSC_PERM &&
>>>> -        fault_status != FSC_ACCESS) {
>>>> +
>>>> +    /* The host kernel will handle the synchronous external abort. There
>>>> +     * is no need to pass the error into the guest.
>>>> +     */
>>>> +    if (is_abort_synchronous(fault_status)) {
>>>> +        if(handle_guest_sea((unsigned long)fault_ipa,
>>>> +                    kvm_vcpu_get_hsr(vcpu))) {
>>> ... Looking further up in this function:
>>>>      is_iabt = kvm_vcpu_trap_is_iabt(vcpu);
>>>>      if (unlikely(!is_iabt && kvm_vcpu_dabt_isextabt(vcpu))) {
>>>>          kvm_inject_vabt(vcpu);
>>>>          return 1;
>>>>      }
>>> ... so external data aborts will have already been 'claimed' by kvm and dealt
>>> with, and we already have a helper for spotting external aborts. (sorry I didn't
>>> spot it earlier).
>>>
>>> We need to do the handle_guest_sea() before this code.
>>>
>>> kvm_inject_vabt() makes an SError interrupt pending for the guest. This makes a
>>> synchronous error asynchronous as the guest may have SError interrupts masked.
>>>
>>> I guess this was the best that could be done at the time of (4055710baca8
>>> "arm/arm64: KVM: Inject virtual abort when guest exits on external abort"), but
>>> in the light of this firmware-first handling, I'm not sure its the right thing
>>> to do.
>>>
>>> Is it possible for handle_guest_sea() to return whether it actually found any
>>> work to do? If there was none I think we should keep this kvm_inject_vabt() as
>>> it is the existing behaviour.
>> Yes, I'll move the handle_guest_sea() call above this. My testing didn't call
>> into that if statement for some reason...it made it to the handle_guest_sea()
>> call successfully.
> I guess you're not using data aborts for testing then.
>
>
>> If there is no work for the GHES code to do it will return and could still make
>> the kvm_inject_vabt() call. It will also return and do that same thing if the
>> error was not fatal in GHES...would that be an issue?
> We might inject a superfluous SError Interrupt in that case.
>
> For memory errors we may do the whole unmapping and signalling thing to handle
> the fault. For recoverable faults, QEMU can generate its own CPER records for
> the guest and do the work to notify the guest. Everything looks fine until the
> guest gets the extra SError interrupt.
>
> If there is firmware-first RAS data, we should skip the injected SError
> Interrupt as the host's RAS code should choose what to do. If this Synchronous
> External Abort was nothing to do with RAS, we should inject the SError interrupt
> as it's the existing behaviour, and not all platforms will have this Synchronous
> External Abort mechanism.
>
> x86's APEI NMI needs to know if the NMI was due to RAS, so we can probably
> borrow the same trick. ghes_notify_nmi() calls ghes_read_estatus() for each
> struct ghes. If they all return an error, there was no work to do.
>
>
> (I assume firmware will only generate one of these at a time, so there is no
> risk of one list-walker processing two entries, then the second finding nothing
> to do?)
>
Okay, I will add in this functionality to avoid injecting an SEI if 
there is actually a GHES error populated.

Firmware that supports the new specs should only generate one of these 
at a time, it will wait for the ack from kernel before sending a second 
error (patch 1 of this series).

Thanks,
Tyler
>
>
>>>> +            kvm_err("Failed to handle guest SEA, FSC: EC=%#x xFSC=%#lx
>>>> ESR_EL2=%#lx\n",
>>>> +                kvm_vcpu_trap_get_class(vcpu),
>>>> +                (unsigned long)kvm_vcpu_trap_get_fault(vcpu),
>>>> +                (unsigned long)kvm_vcpu_get_hsr(vcpu));
>>>> +            return -EFAULT;
>>>> +        }
>>>> +    } else if (fault_status != FSC_FAULT && fault_status != FSC_PERM &&
>>>> +           fault_status != FSC_ACCESS) {
>>>>            kvm_err("Unsupported FSC: EC=%#x xFSC=%#lx ESR_EL2=%#lx\n",
>>>>                kvm_vcpu_trap_get_class(vcpu),
>>>>                (unsigned long)kvm_vcpu_trap_get_fault(vcpu),

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