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Date:   Thu, 21 Oct 2021 11:57:55 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Xuesong Chen <xuesong.chen@...ux.alibaba.com>
Cc:     catalin.marinas@....com, lorenzo.pieralisi@....com,
        james.morse@....com, will@...nel.org, rafael@...nel.org,
        tony.luck@...el.com, bp@...en8.de, mingo@...nel.org,
        bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        Gong <gong.chen@...ux.intel.com>
Subject: Re: [PATCH v3 2/2] ACPI: APEI: Filter the PCI MCFG address with an
 arch-agnostic method

On Thu, Oct 21, 2021 at 11:46:40PM +0800, Xuesong Chen wrote:
> On 21/10/2021 02:50, Bjorn Helgaas wrote:
> > On Wed, Oct 20, 2021 at 11:16:38AM +0800, Xuesong Chen wrote:
> >> On 20/10/2021 03:23, Bjorn Helgaas wrote:
> >>> On Tue, Oct 19, 2021 at 12:50:33PM +0800, Xuesong Chen wrote:

> >>>> This patch will try to handle this case in a more common way
> >>>> instead of the original 'arch' specific solution, which will be
> >>>> beneficial to all the APEI-dependent platforms after that.
> >>>
> >>> This actually doesn't say anything about what the patch does or
> >>> how it works.  It says "handles this case in a more common way"
> >>> but with no details.
> >>
> >> Good suggestion, I'll give more details about that...
> >>
> >>> The EINJ table contains "injection instructions" that can read
> >>> or write "register regions" described by generic address
> >>> structures (see ACPI v6.3, sec 18.6.2 and 18.6.3), and
> >>> __einj_error_trigger() requests those register regions with
> >>> request_mem_region() or request_region() before executing the
> >>> injections instructions.
> >>>
> >>> IIUC, this patch basically says "if this region is part of the
> >>> MCFG area, we don't need to reserve it." That leads to the
> >>> questions of why we need to reserve *any* of the areas
> >>
> >> AFAIK, the MCFG area is reserved since the ECAM module will
> >> provide a generic Kernel Programming Interfaces(KPI), e.g,
> >> pci_generic_config_read(...), so all the drivers are allowed to
> >> access the pci config space only by those KPIs in a consistent
> >> and safe way, direct raw access will break the rule.  Correct me
> >> if I am missing sth.
> >>
> >>> and why it's safe to simply skip reserving regions that are part
> >>> of the MCFG area.
> >>
> >> Actual there is a commit d91525eb8ee6("ACPI, EINJ: Enhance error
> >> injection tolerance level") before to address this issue, the
> >> entire commit log as below:
> >>
> >>     Some BIOSes utilize PCI MMCFG space read/write opertion to trigger
> >>     specific errors. EINJ will report errors as below when hitting such
> >>     cases:
> >>     
> >>     APEI: Can not request [mem 0x83f990a0-0x83f990a3] for APEI EINJ Trigger registers
> >>     
> >>     It is because on x86 platform ACPI based PCI MMCFG logic has
> >>     reserved all MMCFG spaces so that EINJ can't reserve it again.
> >>     We already trust the ACPI/APEI code when using the EINJ interface
> >>     so it is not a big leap to also trust it to access the right
> >>     MMCFG addresses. Skip address checking to allow the access.
> > 
> > I'm not really convinced by that justification because I don't
> > think the issue here is *trust*.  If all we care about is trust,
> > and we trust the ACPI/APEI code, why do we need to reserve
> > anything at all when executing EINJ actions?
> > 
> > I think the resource reservation issue is about coordinating
> > multiple users of the address space.  A driver reserves the MMIO
> > address space of a device it controls so no other driver can
> > reserve it at the same time and cause conflicts.
> > 
> > I'm not really convinced by this mutual exclusion argument either,
> > because I haven't yet seen a situation where we say "EINJ needs a
> > resource that's already in use by somebody else, so we can't use
> > EINJ."  When conflicts arise, the response is always "we'll just
> > stop reserving this conflicting resource but use it anyway."
> > 
> > I think the only real value in apei_resources_request() is a
> > little bit of documentation in /proc/iomem.  For ERST and EINJ,
> > even that only lasts for the tiny period when we're actually
> > executing an action.
> > 
> > So convince me there's a reason why we shouldn't just remove
> > apei_resources_request() completely :)
> 
> I have to confess that currently I have no strong evidence/reason to
> convince you that it's absolute safe to remove
> apei_resources_request(),  probably in some conditions it *does*
> require to follow the mutual exclusion usage model.  The ECAM/MCFG
> maybe a special case not like other normal device driver, since all
> its MCFG space has been reserved during the initialization. Anyway,
> it's another topic and good point well worth discussing in the
> future.

This is missing the point.  It's not the MCFG reservation during
initialization that would make this safe.  What would make it safe is
the fact that ECAM does not require mutual exclusion.

When the hardware implements ECAM correctly, PCI config accesses do
not require locking because a config access requires a single MMIO
load or store.

Many non-ECAM config accessors *do* require locking because they use
several register accesses, e.g., the 0xCF8/0xCFC address/data pairs
used by pci_conf1_read().  If EINJ actions used these, we would have
to enforce mutual exclusion between EINJ config accesses and those
done by other drivers.

Some ARM64 platforms do not implement ECAM correctly, e.g.,
tegra194_map_bus() programs an outbound ATU and xgene_pcie_map_bus()
sets an RTDID register before the MMIO load/store.  Platforms like
this *do* require mutual exclusion between an EINJ config access and
other config accesses.

These platforms are supported via quirks in pci_mcfg.c, so they will
have resources in the pci_mcfg_list, and if we just ignore all the
MCFG resources in apei_resources_request(), there will be nothing to
prevent ordinary driver config accesses from being corrupted by EINJ
accesses.

I think in general, is probably *is* safe to remove MCFG resources
from the APEI reservations, but it would be better if we had some way
to prevent EINJ from using MCFG on platforms like tegra194 and xgene.

> From the patch set itself, I don't think it's a nice idea to make a
> dramatic change regarding the apei_resources_request() part, I
> suggest to keep the original rationale untouched and based on that
> to fix the real issue at hand in a more generic way.

There *was* no original rationale.  The whole point of this
conversation is to figure out what the real rationale is.

> >> Except that the above explanation, IMO the EINJ is only a RAS
> >> debug framework, in this code path, sometimes we need to acesss
> >> the address within the MCFG space directly to trigger kind of HW
> >> error, which behavior does not like the normal device driver's,
> >> in this case some possible unsafe operations (bypass the ecam
> >> ops) can be mitigated because the touched device will generate
> >> some HW errors and the RAS handling part will preempt its
> >> corresponding drivers to fix/log the HW error, that's my
> >> understanding about that.
> > 
> >>>> Signed-off-by: Xuesong Chen <xuesong.chen@...ux.alibaba.com>
> >>>> Reported-by: kernel test robot <lkp@...el.com>
> >>>> Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> >>>> Cc: Catalin Marinas <catalin.marinas@....com>
> >>>> Cc: James Morse <james.morse@....com>
> >>>> Cc: Will Deacon <will@...nel.org>
> >>>> Cc: Rafael. J. Wysocki <rafael@...nel.org>
> >>>> Cc: Tony Luck <tony.luck@...el.com>
> >>>> Cc: Tomasz Nowicki <tn@...ihalf.com>
> >>>> ---
> >>>>  arch/x86/pci/mmconfig-shared.c | 28 --------------------------
> >>>>  drivers/acpi/apei/apei-base.c  | 45 ++++++++++++++++++++++++++++--------------
> >>>>  2 files changed, 30 insertions(+), 43 deletions(-)
> >>>>
> >>>> diff --git a/arch/x86/pci/mmconfig-shared.c b/arch/x86/pci/mmconfig-shared.c
> >>>> index 0b961fe6..12f7d96 100644
> >>>> --- a/arch/x86/pci/mmconfig-shared.c
> >>>> +++ b/arch/x86/pci/mmconfig-shared.c
> >>>> @@ -605,32 +605,6 @@ static int __init pci_parse_mcfg(struct acpi_table_header *header)
> >>>>  	return 0;
> >>>>  }
> >>>>  
> >>>> -#ifdef CONFIG_ACPI_APEI
> >>>> -extern int (*arch_apei_filter_addr)(int (*func)(__u64 start, __u64 size,
> >>>> -				     void *data), void *data);
> >>>> -
> >>>> -static int pci_mmcfg_for_each_region(int (*func)(__u64 start, __u64 size,
> >>>> -				     void *data), void *data)
> >>>> -{
> >>>> -	struct pci_mmcfg_region *cfg;
> >>>> -	int rc;
> >>>> -
> >>>> -	if (list_empty(&pci_mmcfg_list))
> >>>> -		return 0;
> >>>> -
> >>>> -	list_for_each_entry(cfg, &pci_mmcfg_list, list) {
> >>>> -		rc = func(cfg->res.start, resource_size(&cfg->res), data);
> >>>> -		if (rc)
> >>>> -			return rc;
> >>>> -	}
> >>>> -
> >>>> -	return 0;
> >>>> -}
> >>>> -#define set_apei_filter() (arch_apei_filter_addr = pci_mmcfg_for_each_region)
> >>>> -#else
> >>>> -#define set_apei_filter()
> >>>> -#endif
> >>>> -
> >>>>  static void __init __pci_mmcfg_init(int early)
> >>>>  {
> >>>>  	pci_mmcfg_reject_broken(early);
> >>>> @@ -665,8 +639,6 @@ void __init pci_mmcfg_early_init(void)
> >>>>  		else
> >>>>  			acpi_table_parse(ACPI_SIG_MCFG, pci_parse_mcfg);
> >>>>  		__pci_mmcfg_init(1);
> >>>> -
> >>>> -		set_apei_filter();
> >>>>  	}
> >>>>  }
> >>>>  
> >>>> diff --git a/drivers/acpi/apei/apei-base.c b/drivers/acpi/apei/apei-base.c
> >>>> index c7fdb12..daae75a 100644
> >>>> --- a/drivers/acpi/apei/apei-base.c
> >>>> +++ b/drivers/acpi/apei/apei-base.c
> >>>> @@ -21,6 +21,7 @@
> >>>>  #include <linux/kernel.h>
> >>>>  #include <linux/module.h>
> >>>>  #include <linux/init.h>
> >>>> +#include <linux/pci.h>
> >>>>  #include <linux/acpi.h>
> >>>>  #include <linux/slab.h>
> >>>>  #include <linux/io.h>
> >>>> @@ -448,13 +449,34 @@ static int apei_get_nvs_resources(struct apei_resources *resources)
> >>>>  	return acpi_nvs_for_each_region(apei_get_res_callback, resources);
> >>>>  }
> >>>>  
> >>>> -int (*arch_apei_filter_addr)(int (*func)(__u64 start, __u64 size,
> >>>> -				     void *data), void *data);
> >>>> -static int apei_get_arch_resources(struct apei_resources *resources)
> >>>> +#ifdef CONFIG_PCI
> >>>> +extern struct list_head pci_mmcfg_list;
> >>>> +static int apei_filter_mcfg_addr(struct apei_resources *res,
> >>>> +			struct apei_resources *mcfg_res)
> >>>> +{
> >>>> +	int rc = 0;
> >>>> +	struct pci_mmcfg_region *cfg;
> >>>> +
> >>>> +	if (list_empty(&pci_mmcfg_list))
> >>>> +		return 0;
> >>>> +
> >>>> +	apei_resources_init(mcfg_res);
> >>>> +	list_for_each_entry(cfg, &pci_mmcfg_list, list) {
> >>>> +		rc = apei_res_add(&mcfg_res->iomem, cfg->res.start, resource_size(&cfg->res));
> >>>> +		if (rc)
> >>>> +			return rc;
> >>>> +	}
> >>>>  
> >>>> +	/* filter the mcfg resource from current APEI's */
> >>>> +	return apei_resources_sub(res, mcfg_res);
> >>>> +}
> >>>> +#else
> >>>> +static inline int apei_filter_mcfg_addr(struct apei_resources *res,
> >>>> +			struct apei_resources *mcfg_res)
> >>>>  {
> >>>> -	return arch_apei_filter_addr(apei_get_res_callback, resources);
> >>>> +	return 0;
> >>>>  }
> >>>> +#endif
> >>>>  
> >>>>  /*
> >>>>   * IO memory/port resource management mechanism is used to check
> >>>> @@ -486,15 +508,9 @@ int apei_resources_request(struct apei_resources *resources,
> >>>>  	if (rc)
> >>>>  		goto nvs_res_fini;
> >>>>  
> >>>> -	if (arch_apei_filter_addr) {
> >>>> -		apei_resources_init(&arch_res);
> >>>> -		rc = apei_get_arch_resources(&arch_res);
> >>>> -		if (rc)
> >>>> -			goto arch_res_fini;
> >>>> -		rc = apei_resources_sub(resources, &arch_res);
> >>>> -		if (rc)
> >>>> -			goto arch_res_fini;
> >>>> -	}
> >>>> +	rc = apei_filter_mcfg_addr(resources, &arch_res);
> >>>> +	if (rc)
> >>>> +		goto arch_res_fini;
> >>>>  
> >>>>  	rc = -EINVAL;
> >>>>  	list_for_each_entry(res, &resources->iomem, list) {
> >>>> @@ -544,8 +560,7 @@ int apei_resources_request(struct apei_resources *resources,
> >>>>  		release_mem_region(res->start, res->end - res->start);
> >>>>  	}
> >>>>  arch_res_fini:
> >>>> -	if (arch_apei_filter_addr)
> >>>> -		apei_resources_fini(&arch_res);
> >>>> +	apei_resources_fini(&arch_res);
> >>>>  nvs_res_fini:
> >>>>  	apei_resources_fini(&nvs_resources);
> >>>>  	return rc;
> >>>> -- 
> >>>> 1.8.3.1
> >>>>

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