Re: [PATCH v3 1/3] arm64: kvm: support kvmtool to detect RAS extension feature

[gengdongjiu <gengdongjiu@huawei.com>]

Dear, James

On 2017/5/9 1:31, James Morse wrote:
> Hi gengdongjiu,
> 
> On 04/05/17 18:20, gengdongjiu wrote:
>>> On 30/04/17 06:37, Dongjiu Geng wrote:
>>>> Handle kvmtool's detection for RAS extension, because sometimes
>>>> the APP needs to know the CPU's capacity
>>>
>>>> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
>>>> index d9e9697..1004039 100644
>>>> --- a/arch/arm64/kvm/reset.c
>>>> +++ b/arch/arm64/kvm/reset.c
>>>> @@ -64,6 +64,14 @@ static bool cpu_has_32bit_el1(void)
>>>>   return !!(pfr0 & 0x20);
>>>>  }
>>>>
>>>> +static bool kvm_arm_support_ras_extension(void)
>>>> +{
>>>> + u64 pfr0;
>>>> +
>>>> + pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
>>>> + return !!(pfr0 & 0x10000000);
>>>> +}
>>>
>>> Why are we telling user-space that the CPU has RAS extensions? EL0 can't do
>>> anything with this and the guest EL1 can detect it from the id registers.
>>>
>>>
>>> Are you using this to decide whether or not to generate a HEST for the guest?
>>
>> James, yes, it is.  my current user-space qemu EL0 patches indeed will
>> check the RAS  extensions.
>> if has the RAS extensions. for SEA, userspace qemu will generate the
>> CPER and inject the SEA to guest;
>> for SEI,  userspace qemu sets the virtual SEI with the specified
>> Syndrome(set the HCR_EL2.VSE and vsesr_el2 );
>> if not have RAS extensions, Qemu does nothing
> 
> But you can use APEI in a guest on CPUs without the RAS extensions: the host may
> signal memory errors to Qemu for any number of reasons, user-space shouldn't
> care how it knows. Examples are PCI-AER, any APEI event notified by polling or
> one of the flavours of irq.
> 
> I would expect Qemu to generate a HEST based on its abilities, i.e. if it
> supports any mechanism of notifying the guest about errors. Choosing the
> mechanism then depends on the type of error.
> 
> Ideally the Qemu code for HEST/GHES/CPER generation code using some of the irqs
> and polling could be shared with x86, as these should be possible using common
> KVM APIs.
Ok, got it.

> 
> 
>>> If Qemu/kvmtool supports handling memory-failure notifications from signals you
>>> should always generate a HEST. The GHES notification method could be anything
>>> Qemu can deliver to the guest using the KVM APIs. Notifications from Qemu to the
>>> guest don't depend on the RAS extensions. KVM has APIs for IRQ and SEA (you can
>>> use KVM_SET_ONE_REG).
>>
>> I will consider your suggestion to  always generate a CPER instead of
> 
> (generate a HEST, CPER are the runtime records. There are too many acronyms in
> this space!)
  thanks James's correction.

> 
>> relying on the RAS extensions, thanks
> 
> 
> Thanks,
> 
> James
> 
> 
> .
>