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

["Duan, Zhenzhong" <zhenzhong.duan@intel.com>]

Hi Jonathan,

>-----Original Message-----
>From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
>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" <zhenzhong.duan@intel.com> wrote:
>
>> >-----Original Message-----
>> >From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
>> >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 <zhenzhong.duan@intel.com> 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 <yudong.wang@intel.com>
>> >> Co-developed-by: "Wang, Qingshun" <qingshun.wang@linux.intel.com>
>> >> Signed-off-by: "Wang, Qingshun" <qingshun.wang@linux.intel.com>
>> >> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
>> >> ---
>> >>  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😊

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 ||
>>
>>