Message-ID: <8b9d254a-5450-d841-baf7-5819a88043e4@codeaurora.org>
Date:   Fri, 20 Jan 2017 13:58:58 -0700
From:   "Baicar, Tyler" <tbaicar@...eaurora.org>
To:     James Morse <james.morse@....com>
Cc:     christoffer.dall@...aro.org, marc.zyngier@....com,
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Subject: Re: [PATCH V7 05/10] acpi: apei: handle SEA notification type for
 ARMv8

On 1/19/2017 10:57 AM, James Morse wrote:
> Hi Tyler,
>
> On 18/01/17 23:51, Baicar, Tyler wrote:
>> On 1/18/2017 7:50 AM, James Morse wrote:
>>> On 12/01/17 18:15, Tyler Baicar wrote:
>>>> ARM APEI extension proposal added SEA (Synchrounous External
>>>> Abort) notification type for ARMv8.
>>>> Add a new GHES error source handling function for SEA. If an error
>>>> source's notification type is SEA, then this function can be registered
>>>> into the SEA exception handler. That way GHES will parse and report
>>>> SEA exceptions when they occur.
>>>> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
>>>> index 2acbc60..87efe26 100644
>>>> --- a/drivers/acpi/apei/ghes.c
>>>> +++ b/drivers/acpi/apei/ghes.c
>>>> @@ -767,6 +772,62 @@ static struct notifier_block ghes_notifier_sci = {
>>>>        .notifier_call = ghes_notify_sci,
>>>>    };
>>>>    +#ifdef CONFIG_HAVE_ACPI_APEI_SEA
>>>> +static LIST_HEAD(ghes_sea);
>>>> +
>>>> +static int ghes_notify_sea(struct notifier_block *this,
>>>> +                  unsigned long event, void *data)
>>>> +{
>>>> +    struct ghes *ghes;
>>>> +    int ret = NOTIFY_DONE;
>>>> +
>>>> +    nmi_enter();
>>> Can we move this into the arch code? Its because we got here from a
>>> synchronous-exception that makes this nmi-like, I think it only makes sense for
>>> it be called from under /arch/.
>> So move the nmi_enter/exit calls into do_sea of the previous patch? I can do
>> that in the next patchset.
>>> Where did the rcu_read_lock() go? I can see its missing from ghes_notify_nmi()
>>> too, but I don't know enough about RCU to know if that's safe!
>>>
>>> The second paragraph in the comment above rcu_read_lock() describes it as
>>> preventing call_rcu() during a read-side critical section that was running
>>> concurrently. Doesn't this mean we can race with ghes_sea_remove() on another
>>> CPU because we wait for the wrong grace period?
>>>
>>> The same comment talks about how these read-side critical sections can nest, so
>>> I think its quite safe to make these 'lock' calls here.
>> Sorry, I thought we wanted nmi_enter/exit instead of the rcu_read_lock/unlock. I
>> guess the rcu locks
>> will not cause the deadlock scenario you described in the previous patchset if
>> we have the
>> nmi_enter/exit wrapped around the rcu critical section.
> Ah, not instead of, (well, not initially!).
> The nmi_enter()/nmi_exit() thing was to fix the APEI interrupting APEI problem.
> This is only a problem for notification types which can interrupt
> interrupts-masked code, of which SEA is one. (and x86's NMI is the other).
>
> I think I've found the answer to why the rcu_read_lock() isn't needed.
> synchronize_sched() has:
>> * This means that all preempt_disable code sequences, including NMI and
>> * non-threaded hardware-interrupt handlers, in progress on entry will
>> * have completed before this primitive returns.
> synchronize_rcu() has the same innards, so I'm convinced this its safe not to
> have those calls in here. Could we have a comment along the lines of:
>> synchronize_rcu() will wait for nmi_exit(), so no need to rcu_read_lock().
Okay, I'll add the comment in the next patchset.
> (The more I learn about RCU the scarier it becomes!)
>
>
> There are two other things that need changing to make the in_nmi() code path
> work on arm64.
> Always reserve the virtual-address-space forcing GHES_IOREMAP_PAGES to be 2
> regardless of CONFIG_HAVE_ACPI_APEI_NMI. This is almost revert of
> 594c7255dce7a13cac50cf2470cc56e2c3b0494e (but that did a few other things too).
Looks simple enough, should I force it to 2 in all cases, or add a check 
for CONFIG_HAVE_ACPI_APEI_SEA
similar to the check for CONFIG_HAVE_ACPI_APEI_NMI?
> We also need to fix ghes_ioremap_pfn_nmi() to use arch_apei_get_mem_attribute()
> and not assume PAGE_KERNEL.
So just change the call to ioremap_page_range to:

ioremap_page_range(vaddr, vaddr + PAGE_SIZE, pfn << PAGE_SHIFT, 
arch_apei_get_mem_attribute());

Thanks,
Tyler

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