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Re: [PATCH V15 06/11] acpi: apei: handle SEA notification type for ARMv8

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  Re: [PATCH V15 06/11] acpi: apei: handle SEA notification type for  ARMv8 James Morse <james.morse@arm.com> - 2017-05-08 19:40 +0200
    Re: [PATCH V15 06/11] acpi: apei: handle SEA notification type for  ARMv8 "Baicar, Tyler" <tbaicar@codeaurora.org> - 2017-05-08 22:00 +0200

#1637612 — Re: [PATCH V15 06/11] acpi: apei: handle SEA notification type for ARMv8

Hi Tyler,

On 19/04/17 00:05, Tyler Baicar wrote:
> ARM APEI extension proposal added SEA (Synchronous 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.
> An SEA can interrupt code that had interrupts masked and is treated as
> an NMI. To aid this the page of address space for mapping APEI buffers
> while in_nmi() is always reserved, and ghes_ioremap_pfn_nmi() is
> changed to use the helper methods to find the prot_t to map with in
> the same way as ghes_ioremap_pfn_irq().

> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index b74d8b7..10013ff 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -518,6 +520,17 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>  	pr_err("Synchronous External Abort: %s (0x%08x) at 0x%016lx\n",
>  		inf->name, esr, addr);
>  
> +	/*
> +	 * Synchronous aborts may interrupt code which had interrupts masked.
> +	 * Before calling out into the wider kernel tell the interested
> +	 * subsystems.
> +	 */
> +	if (IS_ENABLED(CONFIG_ACPI_APEI_SEA)) {
> +		nmi_enter();
> +		ghes_notify_sea();
> +		nmi_exit();
> +	}
> +
>  	info.si_signo = SIGBUS;
>  	info.si_errno = 0;
>  	info.si_code  = 0;


I was tidying up the masking/unmasking in entry.S, something I wasn't aware of
that leads to a bug:
entry.S will unmask interrupts for instruction/data aborts that came from a
context with interrupts enabled. This makes sense for get_user() and friends...
For do_sea() we pull nmi_enter() as this can interrupt interrupts-masked code,
such as APEI, but if we end up in here with interrupts unmasked we can take an
IRQ from this 'NMI' context, which will inherit the in_nmi() and could lead to
the deadlock we were originally trying to avoid.

Teaching entry.S to spot external aborts is messy. I think the two choices are
to either mask interrupts when calling nmi_enter() (as these things should be
mutually exclusive), or to conditionally call nmi_enter() based on
interrupts_enabled(regs). I prefer the second one as it matches the notify_sea()
while interruptible that happens when KVM takes one of these.



Thanks,

James

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#1637683

On 5/8/2017 11:28 AM, James Morse wrote:
> Hi Tyler,
>
> On 19/04/17 00:05, Tyler Baicar wrote:
>> ARM APEI extension proposal added SEA (Synchronous 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.
>> An SEA can interrupt code that had interrupts masked and is treated as
>> an NMI. To aid this the page of address space for mapping APEI buffers
>> while in_nmi() is always reserved, and ghes_ioremap_pfn_nmi() is
>> changed to use the helper methods to find the prot_t to map with in
>> the same way as ghes_ioremap_pfn_irq().
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index b74d8b7..10013ff 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -518,6 +520,17 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>>   	pr_err("Synchronous External Abort: %s (0x%08x) at 0x%016lx\n",
>>   		inf->name, esr, addr);
>>   
>> +	/*
>> +	 * Synchronous aborts may interrupt code which had interrupts masked.
>> +	 * Before calling out into the wider kernel tell the interested
>> +	 * subsystems.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_ACPI_APEI_SEA)) {
>> +		nmi_enter();
>> +		ghes_notify_sea();
>> +		nmi_exit();
>> +	}
>> +
>>   	info.si_signo = SIGBUS;
>>   	info.si_errno = 0;
>>   	info.si_code  = 0;
>
> I was tidying up the masking/unmasking in entry.S, something I wasn't aware of
> that leads to a bug:
> entry.S will unmask interrupts for instruction/data aborts that came from a
> context with interrupts enabled. This makes sense for get_user() and friends...
> For do_sea() we pull nmi_enter() as this can interrupt interrupts-masked code,
> such as APEI, but if we end up in here with interrupts unmasked we can take an
> IRQ from this 'NMI' context, which will inherit the in_nmi() and could lead to
> the deadlock we were originally trying to avoid.
>
> Teaching entry.S to spot external aborts is messy. I think the two choices are
> to either mask interrupts when calling nmi_enter() (as these things should be
> mutually exclusive), or to conditionally call nmi_enter() based on
> interrupts_enabled(regs). I prefer the second one as it matches the notify_sea()
> while interruptible that happens when KVM takes one of these.
Hello James,

So it would need to be like this?

         if(interrupts_enabled(regs))
             nmi_enter();
         ghes_notify_sea();
         if(interrupts_enabled(regs))
             nmi_exit();

Thanks,
Tyler

-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.

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