Re: [PATCH] perf/x86/intel/uncore: Fix a kernel WARNING triggered by maxcpus=1

[Peter Zijlstra]

On Wed, May 26, 2021 at 06:58:47AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> A kernel WARNING may be triggered when setting maxcpus=1.
> 
> The uncore counters are Die-scope. When probing a PCI device, only the
> BUS information can be retrieved. The uncore driver has to maintain a
> mapping table used to calculate the logical Die ID from a given BUS#.
> 
> Before the patch ba9506be4e40, the mapping table stores the mapping
> information from the BUS# -> a Physical Socket ID. To calculate the
> logical die ID, perf does,
> - In snbep_pci2phy_map_init(), retrieve the BUS# -> a Physical Socket ID
>   from the UBOX PCI configure space.
> - Calculate the mapping information (a BUS# -> a Physical Socket ID) for
>   the other PCI BUS.
> - In the uncore_pci_probe(), get the physical Socket ID from a given BUS
>   and the mapping table.
> - Calculate the logical Die ID
> 
> Since only the logical Die ID is required, with the patch ba9506be4e40,
> the mapping table stores the mapping information from the BUS# -> a
> logical Die ID. Now perf does,
> - In snbep_pci2phy_map_init(), retrieve the BUS# -> a Physical Socket ID
>   from the UBOX PCI configure space.
> - Calculate the logical Die ID
> - Calculate the mapping information (a BUS# -> a logical Die ID) for the
>   other PCI BUS.
> - In the uncore_pci_probe(), get the logical die ID from a given BUS and
>   the mapping table.
> 
> When calculating the logical Die ID, -1 may be returned, especially when
> maxcpus=1. Here, -1 means the logical Die ID is not found. But when
> calculating the mapping information for the other PCI BUS, -1 indicates
> that it's the other PCI BUS that requires the calculation of the
> mapping. The driver will mistakenly do the calculation.
> 
> Uses the -ENODEV to indicate the case which the logical Die ID is not
> found. The driver will not mess up the mapping table anymore.
> 
> Fixes: ba9506be4e40 ("perf/x86/intel/uncore: Store the logical die id
> instead of the physical die id.")

(please don't wrap like that)

> Reported-by: John Donnelly <john.p.donnelly@xxxxxxxxxx>
> Tested-by: John Donnelly <john.p.donnelly@xxxxxxxxxx>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Thanks!