Subject: Re: [PATCH v3 1/3] PCI/AER: Store UNCOR_STATUS bits that might be ANFE in aer_err_info

From: Jonathan Cameron <[email-masked]>
Date: Wed, 1 May 2024 13:24:29 +0100

On Sun, 28 Apr 2024 03:31:11 +0000
"Duan, Zhenzhong" <[email-masked]> wrote:

> Hi Jonathan,
> 
> >-----Original Message-----
> >From: Jonathan Cameron <[email-masked]>
> >Subject: Re: [PATCH v3 1/3] PCI/AER: Store UNCOR_STATUS bits that might
> >be ANFE in aer_err_info
> >
> >On Tue, 23 Apr 2024 02:25:05 +0000
> >"Duan, Zhenzhong" <[email-masked]> wrote:
> >  
> >> >-----Original Message-----
> >> >From: Jonathan Cameron <[email-masked]>
> >> >Subject: Re: [PATCH v3 1/3] PCI/AER: Store UNCOR_STATUS bits that  
> >might  
> >> >be ANFE in aer_err_info
> >> >
> >> >On Wed, 17 Apr 2024 14:14:05 +0800
> >> >Zhenzhong Duan <[email-masked]> wrote:
> >> >  
> >> >> In some cases the detector of a Non-Fatal Error(NFE) is not the most
> >> >> appropriate agent to determine the type of the error. For example,
> >> >> when software performs a configuration read from a non-existent
> >> >> device or Function, completer will send an ERR_NONFATAL Message.
> >> >> On some platforms, ERR_NONFATAL results in a System Error, which
> >> >> breaks normal software probing.
> >> >>
> >> >> Advisory Non-Fatal Error(ANFE) is a special case that can be used
> >> >> in above scenario. It is predominantly determined by the role of the
> >> >> detecting agent (Requester, Completer, or Receiver) and the specific
> >> >> error. In such cases, an agent with AER signals the NFE (if enabled)
> >> >> by sending an ERR_COR Message as an advisory to software, instead of
> >> >> sending ERR_NONFATAL.
> >> >>
> >> >> When processing an ANFE, ideally both correctable error(CE) status and
> >> >> uncorrectable error(UE) status should be cleared. However, there is no
> >> >> way to fully identify the UE associated with ANFE. Even worse, a Fatal
> >> >> Error(FE) or Non-Fatal Error(NFE) may set the same UE status bit as
> >> >> ANFE. Treating an ANFE as NFE will reproduce above mentioned issue,
> >> >> i.e., breaking softwore probing; treating NFE as ANFE will make us
> >> >> ignoring some UEs which need active recover operation. To avoid  
> >clearing  
> >> >> UEs that are not ANFE by accident, the most conservative route is taken
> >> >> here: If any of the FE/NFE Detected bits is set in Device Status, do not
> >> >> touch UE status, they should be cleared later by the UE handler.  
> >Otherwise,  
> >> >> a specific set of UEs that may be raised as ANFE according to the PCIe
> >> >> specification will be cleared if their corresponding severity is Non-Fatal.
> >> >>
> >> >> To achieve above purpose, store UNCOR_STATUS bits that might be  
> >ANFE  
> >> >> in aer_err_info.anfe_status. So that those bits could be printed and
> >> >> processed later.
> >> >>
> >> >> Tested-by: Yudong Wang <[email-masked]>
> >> >> Co-developed-by: "Wang, Qingshun" <[email-masked]>
> >> >> Signed-off-by: "Wang, Qingshun" <[email-masked]>
> >> >> Signed-off-by: Zhenzhong Duan <[email-masked]>
> >> >> ---
> >> >>  drivers/pci/pci.h      |  1 +
> >> >>  drivers/pci/pcie/aer.c | 45  
> >> >++++++++++++++++++++++++++++++++++++++++++  
> >> >>  2 files changed, 46 insertions(+)
> >> >>
> >> >> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> >> >> index 17fed1846847..3f9eb807f9fd 100644
> >> >> --- a/drivers/pci/pci.h
> >> >> +++ b/drivers/pci/pci.h
> >> >> @@ -412,6 +412,7 @@ struct aer_err_info {
> >> >>
> >> >>  	unsigned int status;		/* COR/UNCOR Error Status */
> >> >>  	unsigned int mask;		/* COR/UNCOR Error Mask */
> >> >> +	unsigned int anfe_status;	/* UNCOR Error Status for ANFE */
> >> >>  	struct pcie_tlp_log tlp;	/* TLP Header */
> >> >>  };
> >> >>
> >> >> diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
> >> >> index ac6293c24976..27364ab4b148 100644
> >> >> --- a/drivers/pci/pcie/aer.c
> >> >> +++ b/drivers/pci/pcie/aer.c
> >> >> @@ -107,6 +107,12 @@ struct aer_stats {
> >> >>  					PCI_ERR_ROOT_MULTI_COR_RCV |  
> >> >	\  
> >> >>  					PCI_ERR_ROOT_MULTI_UNCOR_RCV)
> >> >>
> >> >> +#define AER_ERR_ANFE_UNC_MASK  
> >> >	(PCI_ERR_UNC_POISON_TLP |	\  
> >> >> +					PCI_ERR_UNC_COMP_TIME |  
> >> >	\  
> >> >> +					PCI_ERR_UNC_COMP_ABORT |  
> >> >	\  
> >> >> +					PCI_ERR_UNC_UNX_COMP |  
> >> >	\  
> >> >> +					PCI_ERR_UNC_UNSUP)
> >> >> +
> >> >>  static int pcie_aer_disable;
> >> >>  static pci_ers_result_t aer_root_reset(struct pci_dev *dev);
> >> >>
> >> >> @@ -1196,6 +1202,41 @@ void aer_recover_queue(int domain,  
> >unsigned  
> >> >int bus, unsigned int devfn,  
> >> >>  EXPORT_SYMBOL_GPL(aer_recover_queue);
> >> >>  #endif
> >> >>
> >> >> +static void anfe_get_uc_status(struct pci_dev *dev, struct  
> >aer_err_info  
> >> >*info)  
> >> >> +{
> >> >> +	u32 uncor_mask, uncor_status;
> >> >> +	u16 device_status;
> >> >> +	int aer = dev->aer_cap;
> >> >> +
> >> >> +	if (pcie_capability_read_word(dev, PCI_EXP_DEVSTA,  
> >> >&device_status))  
> >> >> +		return;
> >> >> +	/*
> >> >> +	 * Take the most conservative route here. If there are
> >> >> +	 * Non-Fatal/Fatal errors detected, do not assume any
> >> >> +	 * bit in uncor_status is set by ANFE.
> >> >> +	 */
> >> >> +	if (device_status & (PCI_EXP_DEVSTA_NFED | PCI_EXP_DEVSTA_FED))
> >> >> +		return;
> >> >> +  
> >> >
> >> >Is there not a race here?  If we happen to get either an NFED or FED
> >> >between the read of device_status above and here we might pick up a  
> >status  
> >> >that corresponds to that (and hence clear something we should not).  
> >>
> >> In this scenario, info->anfe_status is 0.  
> >
> >OK. In that case what is the point of the check above?
> >If the code is safe to races, it's safe to go ahead without that check
> >on what might race.  
> 
> Good question.
> After further digging into the spec, I just found I misunderstood it.
> An UNCUR error raised as ANFE can be raised as NFE in different cases,
> so info->anfe_status can be nonzero here and the race you mentioned
> does exist, the check on PCI_EXP_DEVSTA_FED is also unnecessary.
> Sorry for the misleading. I plan to have below change to fix the race:
> 
>        unsigned int anfe_status;
>        anfe_status = uncor_status & ~uncor_mask & ~info->severity &
>                            AER_ERR_ANFE_UNC_MASK;
> 
>        if (pcie_capability_read_word(dev, PCI_EXP_DEVSTA, &device_status))
>                return;
>        /*
>         * Take the most conservative route here. If there are
>         * Non-Fatal errors detected, do not assume any
>         * bit in uncor_status is set by ANFE.
>         */
>        if (device_status & PCI_EXP_DEVSTA_NFED)
>                return;
>         info->anfe_status = anfe_status;
> 
> With this change, there is still a small window between reading uncor_status
> and device_status to leak ANFE, but that's the best we can do and better
> than clearing NFE. Let me know if you have better idea😊

Worth leaving some breadcrumbs about there being a race (so a comment)
and explain what the side effects of hitting that race are (lost info
on the error I think, but not a missed error)?
> 
> Thanks
> Zhenzhong
> 
> >  
> >>  
> >> >
> >> >Or am I missing that race being close somewhere?  
> >>
> >> The bits leading to NFED or FED is masked out when assigning info-
> >>anfe_status.
> >> Bits for FED is masked out by ~info->severity,
> >> bit for NFED is masked out by AER_ERR_ANFE_UNC_MASK.
> >>
> >> So we never clear status bits for NFED or FED in ANFE handler.
> >>
> >> See below assignment of info->anfe_status.
> >>
> >> Thanks
> >> Zhenzhong
> >>  
> >> >  
> >> >> +	pci_read_config_dword(dev, aer + PCI_ERR_UNCOR_STATUS,  
> >> >&uncor_status);  
> >> >> +	pci_read_config_dword(dev, aer + PCI_ERR_UNCOR_MASK,  
> >> >&uncor_mask);  
> >> >> +	/*
> >> >> +	 * According to PCIe Base Specification Revision 6.1,
> >> >> +	 * Section 6.2.3.2.4, if an UNCOR error is raised as
> >> >> +	 * Advisory Non-Fatal error, it will match the following
> >> >> +	 * conditions:
> >> >> +	 *	a. The severity of the error is Non-Fatal.
> >> >> +	 *	b. The error is one of the following:
> >> >> +	 *		1. Poisoned TLP           (Section 6.2.3.2.4.3)
> >> >> +	 *		2. Completion Timeout     (Section 6.2.3.2.4.4)
> >> >> +	 *		3. Completer Abort        (Section 6.2.3.2.4.1)
> >> >> +	 *		4. Unexpected Completion  (Section 6.2.3.2.4.5)
> >> >> +	 *		5. Unsupported Request    (Section 6.2.3.2.4.1)
> >> >> +	 */
> >> >> +	info->anfe_status = uncor_status & ~uncor_mask & ~info->severity  
> >> >&  
> >> >> +			    AER_ERR_ANFE_UNC_MASK;
> >> >> +}
> >> >> +
> >> >>  /**
> >> >>   * aer_get_device_error_info - read error status from dev and store it  
> >to  
> >> >info  
> >> >>   * @dev: pointer to the device expected to have a error record
> >> >> @@ -1213,6 +1254,7 @@ int aer_get_device_error_info(struct pci_dev  
> >> >*dev, struct aer_err_info *info)  
> >> >>
> >> >>  	/* Must reset in this function */
> >> >>  	info->status = 0;
> >> >> +	info->anfe_status = 0;
> >> >>  	info->tlp_header_valid = 0;
> >> >>
> >> >>  	/* The device might not support AER */
> >> >> @@ -1226,6 +1268,9 @@ int aer_get_device_error_info(struct pci_dev  
> >> >*dev, struct aer_err_info *info)  
> >> >>  			&info->mask);
> >> >>  		if (!(info->status & ~info->mask))
> >> >>  			return 0;
> >> >> +
> >> >> +		if (info->status & PCI_ERR_COR_ADV_NFAT)
> >> >> +			anfe_get_uc_status(dev, info);
> >> >>  	} else if (type == PCI_EXP_TYPE_ROOT_PORT ||
> >> >>  		   type == PCI_EXP_TYPE_RC_EC ||
> >> >>  		   type == PCI_EXP_TYPE_DOWNSTREAM ||  
> >>
> >>  
>