[PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[Christophe Lombard]

The cxl driver cannot disable the interrupt at the device level and has
to use disable_irq[_nosync] instead.
To avoid the implementation of the lazy optimisation (the interrupt is
marked disabled, but the hardware is left unmasked), we can disable it,
for a particular irq line, by calling
'irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY)'.

Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
---
 drivers/misc/cxl/guest.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
index f58b4b6c..dc476e1 100644
--- a/drivers/misc/cxl/guest.c
+++ b/drivers/misc/cxl/guest.c
@@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
 		hwirq = ctx->irqs.offset[r];
 		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
 			virq = irq_find_mapping(NULL, hwirq);
+			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
 			disable_irq(virq);
 		}
 	}
-- 
2.7.4

Re: [PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[Benjamin Herrenschmidt]

On Thu, 2018-03-22 at 17:37 +0100, Christophe Lombard wrote:
> The cxl driver cannot disable the interrupt at the device level and has
> to use disable_irq[_nosync] instead.
> To avoid the implementation of the lazy optimisation (the interrupt is
> marked disabled, but the hardware is left unmasked), we can disable it,
> for a particular irq line, by calling
> 'irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY)'.

Why do you need that ? What's wrong with the lazy approach ? It makes
disable_irq/enable_irq faster...

You shouldn't need that unless your device is generating a *LOT* of
irqs while disabled.

> Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
> ---
>  drivers/misc/cxl/guest.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
> index f58b4b6c..dc476e1 100644
> --- a/drivers/misc/cxl/guest.c
> +++ b/drivers/misc/cxl/guest.c
> @@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
>  		hwirq = ctx->irqs.offset[r];
>  		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
>  			virq = irq_find_mapping(NULL, hwirq);
> +			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
>  			disable_irq(virq);
>  		}
>  	}

Re: [PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[christophe lombard]

Le 23/03/2018 à 03:14, Benjamin Herrenschmidt a écrit :
> On Thu, 2018-03-22 at 17:37 +0100, Christophe Lombard wrote:
>> The cxl driver cannot disable the interrupt at the device level and has
>> to use disable_irq[_nosync] instead.
>> To avoid the implementation of the lazy optimisation (the interrupt is
>> marked disabled, but the hardware is left unmasked), we can disable it,
>> for a particular irq line, by calling
>> 'irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY)'.
> 
> Why do you need that ? What's wrong with the lazy approach ? It makes
> disable_irq/enable_irq faster...
> 
> You shouldn't need that unless your device is generating a *LOT* of
> irqs while disabled.
> 

An issue on POWERVM (CAPI) has been introduced with the following patch
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bf22ff45bed664aefb5c4e43029057a199b7070c 

The PSL or AFU interrupts are never received by the cxl driver because 
the interrupts are never unmasked.

Without this patch (genirq: Avoid unnecessary low level irq function 
calls), the callback desc->irq_data.chip->irq_unmask(&desc->irq_data); 
(= ics_rtas_unmask_irq()) is called by default through irq_enable().

The cxl driver disables the interrupts before attaching the process 
element and enables the interrupts after that.

In the current code, irq_enable() unmasks the irq only if the irq state 
is IRQD_IRQ_MASKED but it does not.

Call irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY) allows forcing 
irq_disable() to update the irq state to IRQD_IRQ_MASKED and by default
irq_enable() will unmask the irq through ics_rtas_unmask_irq().




>> Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
>> ---
>>   drivers/misc/cxl/guest.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
>> index f58b4b6c..dc476e1 100644
>> --- a/drivers/misc/cxl/guest.c
>> +++ b/drivers/misc/cxl/guest.c
>> @@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
>>   		hwirq = ctx->irqs.offset[r];
>>   		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
>>   			virq = irq_find_mapping(NULL, hwirq);
>> +			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
>>   			disable_irq(virq);
>>   		}
>>   	}

Re: [PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[Benjamin Herrenschmidt]

On Fri, 2018-03-23 at 17:17 +0100, christophe lombard wrote:
> Le 23/03/2018 à 03:14, Benjamin Herrenschmidt a écrit :
> > On Thu, 2018-03-22 at 17:37 +0100, Christophe Lombard wrote:
> > > The cxl driver cannot disable the interrupt at the device level and has
> > > to use disable_irq[_nosync] instead.
> > > To avoid the implementation of the lazy optimisation (the interrupt is
> > > marked disabled, but the hardware is left unmasked), we can disable it,
> > > for a particular irq line, by calling
> > > 'irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY)'.
> > 
> > Why do you need that ? What's wrong with the lazy approach ? It makes
> > disable_irq/enable_irq faster...
> > 
> > You shouldn't need that unless your device is generating a *LOT* of
> > irqs while disabled.
> > 
> 
> An issue on POWERVM (CAPI) has been introduced with the following patch
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bf22ff45bed664aefb5c4e43029057a199b7070c 
> 
> The PSL or AFU interrupts are never received by the cxl driver because 
> the interrupts are never unmasked.
> 
> Without this patch (genirq: Avoid unnecessary low level irq function 
> calls), the callback desc->irq_data.chip->irq_unmask(&desc->irq_data); 
> (= ics_rtas_unmask_irq()) is called by default through irq_enable().

I don't see why this would change with the patch... 

> The cxl driver disables the interrupts before attaching the process 
> element and enables the interrupts after that.

How ? Using disable_irq or something else ?

> In the current code, irq_enable() unmasks the irq only if the irq state 
> is IRQD_IRQ_MASKED but it does not.

Sorry I don't really parse your sentence. You mean there's a disconnect
between the "HW" (or pHyp) state of the interrupt and the
IRQD_IRQ_MASKED flag ? That shouldn't happen... if that's the case the
bug is elsewhere, what is causing the disconnect in the first place ?

> Call irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY) allows forcing 
> irq_disable() to update the irq state to IRQD_IRQ_MASKED and by default
> irq_enable() will unmask the irq through ics_rtas_unmask_irq().

Hrm. I don't quite understand. You shouldn't need that, I suspect you
are papering over a different bug but I'm not 100% certain as I don't
completely understand what's happening.

Cheers,
Ben.
> 
> 
> 
> > > Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
> > > ---
> > >   drivers/misc/cxl/guest.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
> > > index f58b4b6c..dc476e1 100644
> > > --- a/drivers/misc/cxl/guest.c
> > > +++ b/drivers/misc/cxl/guest.c
> > > @@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
> > >   		hwirq = ctx->irqs.offset[r];
> > >   		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
> > >   			virq = irq_find_mapping(NULL, hwirq);
> > > +			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
> > >   			disable_irq(virq);
> > >   		}
> > >   	}

Re: [PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[christophe lombard]

Le 24/03/2018 à 09:14, Benjamin Herrenschmidt a écrit :
> On Fri, 2018-03-23 at 17:17 +0100, christophe lombard wrote:
>> Le 23/03/2018 à 03:14, Benjamin Herrenschmidt a écrit :
>>> On Thu, 2018-03-22 at 17:37 +0100, Christophe Lombard wrote:
>>>> The cxl driver cannot disable the interrupt at the device level and has
>>>> to use disable_irq[_nosync] instead.
>>>> To avoid the implementation of the lazy optimisation (the interrupt is
>>>> marked disabled, but the hardware is left unmasked), we can disable it,
>>>> for a particular irq line, by calling
>>>> 'irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY)'.
>>>
>>> Why do you need that ? What's wrong with the lazy approach ? It makes
>>> disable_irq/enable_irq faster...
>>>
>>> You shouldn't need that unless your device is generating a *LOT* of
>>> irqs while disabled.
>>>
>>
>> An issue on POWERVM (CAPI) has been introduced with the following patch
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bf22ff45bed664aefb5c4e43029057a199b7070c
>>
>> The PSL or AFU interrupts are never received by the cxl driver because
>> the interrupts are never unmasked.
>>
>> Without this patch (genirq: Avoid unnecessary low level irq function
>> calls), the callback desc->irq_data.chip->irq_unmask(&desc->irq_data);
>> (= ics_rtas_unmask_irq()) is called by default through irq_enable().
> 
> I don't see why this would change with the patch...
> 
>> The cxl driver disables the interrupts before attaching the process
>> element and enables the interrupts after that.
> 
> How ? Using disable_irq or something else ?
> 
>> In the current code, irq_enable() unmasks the irq only if the irq state
>> is IRQD_IRQ_MASKED but it does not.
> 
> Sorry I don't really parse your sentence. You mean there's a disconnect
> between the "HW" (or pHyp) state of the interrupt and the
> IRQD_IRQ_MASKED flag ? That shouldn't happen... if that's the case the
> bug is elsewhere, what is causing the disconnect in the first place ?
> 
>> Call irq_set_status_flags(irq, IRQ_DISABLE_UNLAZY) allows forcing
>> irq_disable() to update the irq state to IRQD_IRQ_MASKED and by default
>> irq_enable() will unmask the irq through ics_rtas_unmask_irq().
> 
> Hrm. I don't quite understand. You shouldn't need that, I suspect you
> are papering over a different bug but I'm not 100% certain as I don't
> completely understand what's happening.
> 
> Cheers,
> Ben.

Hi Ben,

To answer to your questions, here is the timeline in the cxl driver

   1. call disable_irq()

   2. call plpar_hcall9() to attach a process element
	During this phase, phyp (as described in CAPI PAPR document) 	
	disables the virtual interrupts provided in the process-element-
	struct.
	Then the partition must use ibm,set-xive to enable the virtual
	interrupt source after H_ATTACH_CA_PROCESS completes
	successfully.

   3. call enable_irq()
	Before the following patch (genirq: Avoid unnecessary low level
	irq function calls) be pushed in mainline, the unmask api
	(= ics_rtas_unmask_irq()) was called by default and everything
	was ok.

	With this patch, the unmask api is now called only if 		
	IRQD_IRQ_MASKED is set and in our case, it's never done and the
	irqs remain masked.


   So, there is a disconnect between the 'HW' (phyp) state of the
   interrupt and the IRQD_IRQ_MASKED flag.

   In the timeline of the cxl driver, the IRQD_IRQ_MASKED flag is never
   set.
   This flag was not used in enable_irq(), before that the patch "genirq:
   Avoid ..." was pushed. So ics_rtas_unmask_irq() was called and
   everything was ok.

Thanks

>>
>>
>>
>>>> Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
>>>> ---
>>>>    drivers/misc/cxl/guest.c | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
>>>> index f58b4b6c..dc476e1 100644
>>>> --- a/drivers/misc/cxl/guest.c
>>>> +++ b/drivers/misc/cxl/guest.c
>>>> @@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
>>>>    		hwirq = ctx->irqs.offset[r];
>>>>    		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
>>>>    			virq = irq_find_mapping(NULL, hwirq);
>>>> +			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
>>>>    			disable_irq(virq);
>>>>    		}
>>>>    	}
> 

Re: [PATCH] cxl: disable the lazy approach for irqs in POWERVM environment.

[Benjamin Herrenschmidt]

On Mon, 2018-05-07 at 18:02 +0200, christophe lombard wrote:
> 
> To answer to your questions, here is the timeline in the cxl driver
> 
>    1. call disable_irq()
> 
>    2. call plpar_hcall9() to attach a process element
> 	During this phase, phyp (as described in CAPI PAPR document) 	
> 	disables the virtual interrupts provided in the process-element-
> 	struct.
> 	Then the partition must use ibm,set-xive to enable the virtual
> 	interrupt source after H_ATTACH_CA_PROCESS completes
> 	successfully.

Ok, this is the breakage. This PAPR API is badly defined, nothing other
than the existing interrupt management PAPR calls should affect the
enable/disable state of the interrupt The fact that attaching a process
element does is a broken design.

>    3. call enable_irq()
> 	Before the following patch (genirq: Avoid unnecessary low level
> 	irq function calls) be pushed in mainline, the unmask api
> 	(= ics_rtas_unmask_irq()) was called by default and everything
> 	was ok.
> 
> 	With this patch, the unmask api is now called only if 		
> 	IRQD_IRQ_MASKED is set and in our case, it's never done and the
> 	irqs remain masked.
> 
> 
>    So, there is a disconnect between the 'HW' (phyp) state of the
>    interrupt and the IRQD_IRQ_MASKED flag.
> 
>    In the timeline of the cxl driver, the IRQD_IRQ_MASKED flag is never
>    set.
>    This flag was not used in enable_irq(), before that the patch "genirq:
>    Avoid ..." was pushed. So ics_rtas_unmask_irq() was called and
>    everything was ok.
> 
> Thanks
> 
> > > 
> > > 
> > > 
> > > > > Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
> > > > > ---
> > > > >    drivers/misc/cxl/guest.c | 1 +
> > > > >    1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/drivers/misc/cxl/guest.c b/drivers/misc/cxl/guest.c
> > > > > index f58b4b6c..dc476e1 100644
> > > > > --- a/drivers/misc/cxl/guest.c
> > > > > +++ b/drivers/misc/cxl/guest.c
> > > > > @@ -389,6 +389,7 @@ static void disable_afu_irqs(struct cxl_context *ctx)
> > > > >    		hwirq = ctx->irqs.offset[r];
> > > > >    		for (i = 0; i < ctx->irqs.range[r]; hwirq++, i++) {
> > > > >    			virq = irq_find_mapping(NULL, hwirq);
> > > > > +			irq_set_status_flags(virq, IRQ_DISABLE_UNLAZY);
> > > > >    			disable_irq(virq);
> > > > >    		}
> > > > >    	}