RE: [PATCH 1/1] x86/platform/x86: Fix count of CHas on multi-pci-segment arches

["Kroening, Gary" <gary.kroening@hpe.com>]

Thanks, Kan -- your patch looks good, and cleaner than the previous method!

Tonight, I've tested it on our in-house simulator for the following configurations. The simulator has been matching hardware well for this issue -- we'll do more testing on real hardware, but I'm confident you have it right.

Terminology:

"hubless" -- equivalent to "glueless" or "white box" where our BIOS sets things up with a single bus for all sockets.

"scalable" -- BIOS assigns a new segment/domain for each socket

Configurations tested so far:
- single-socket hubless/scalable
- two sockets, scalable
- four sockets, hubless
- eight sockets, scalable

In all cases, skx_count_chabox() is returning 28 for Skylake server.
Thanks for the quick response!
Gary

> -----Original Message-----
> From: Liang, Kan [mailto:kan.liang@linux.intel.com]
> Sent: Monday, March 12, 2018 8:43 PM
> To: Kroening, Gary; mingo@redhat.com; hpa@zytor.com; tglx@linutronix.de;
> peterz@infradead.org
> Cc: Travis, Mike; Banman, Andrew; Sivanich, Dimitri; Anderson, Russ;
> x86@kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 1/1] x86/platform/x86: Fix count of CHas on multi-pci-
> segment arches
> 
> 
> 
> On 3/7/2018 3:33 PM, Kroening, Gary wrote:
> > For systems with a single PCI segment, it is sufficient to look for the
> > bus number to change in order to determine that all of the CHa's have
> > been counted for a single socket.
> >
> > However, for multi PCI segment systems, each socket is given a new
> > segment and the bus number does NOT change.  So looking only for the
> > bus number to change ends up counting all of the CHa's on all sockets
> > in the system.  This leads to writing CPU MSRs beyond a valid range and
> > causes an error in ivbep_uncore_msr_init_box().
> >
> > The fix is to check for either the bus number or segment number to
> change.
> >
> 
> Hi Gary,
> 
> There is a recommended way in uncore document to query the number of
> CHAs on Skylake server.
> I have a patch to implement the new way.
> 
> Could you please take a look at the patch and see if it can fix your
> issue?
> 
> 
> Thanks,
> Kan
> 
> ------
>  From 55f54b2fa3021c691c2fd4f5cfc8f441fd104e91 Mon Sep 17 00:00:00 2001
> From: Kan Liang <kan.liang@linux.intel.com>
> Date: Mon, 12 Mar 2018 13:03:40 -0700
> Subject: [PATCH] perf/x86/intel/uncore: Querying number of CHAs from
> CAPID6 register
> 
> The number of CHAs is miscalculated on multi PCI domain systems on
> Skylake server
> 
> (From Kroening, Gary:
> 
> For systems with a single PCI segment, it is sufficient to look for the
> bus number to change in order to determine that all of the CHa's have
> been counted for a single socket.
> However, for multi PCI segment systems, each socket is given a new
> segment and the bus number does NOT change.  So looking only for the
> bus number to change ends up counting all of the CHa's on all sockets
> in the system.  This leads to writing CPU MSRs beyond a valid range and
> causes an error in ivbep_uncore_msr_init_box().)
> 
> To determine the number of CHAs, it should read bits 27:0 in the CAPID6
> register located at Device 30, Function 3, Offset 0x9C. These 28 bits
> form a bit vector of available LLC slices and the CHAs that manage those
> slices.
> 
> Fixes: cd34cd97b7b4 ("perf/x86/intel/uncore: Add Skylake server uncore
> support")
> Reported-by: Kroening, Gary <gary.kroening@hpe.com>
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---
>   arch/x86/events/intel/uncore_snbep.c | 24 ++++++++++--------------
>   1 file changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/events/intel/uncore_snbep.c
> b/arch/x86/events/intel/uncore_snbep.c
> index d4672ed..a42715b 100644
> --- a/arch/x86/events/intel/uncore_snbep.c
> +++ b/arch/x86/events/intel/uncore_snbep.c
> @@ -3575,24 +3575,20 @@ static struct intel_uncore_type
> *skx_msr_uncores[] = {
>   	NULL,
>   };
> 
> +#define SKX_CAPID6		0x9c
> +#define SKX_CHA_BIT_WIDTH	28
> +
>   static int skx_count_chabox(void)
>   {
> -	struct pci_dev *chabox_dev = NULL;
> -	int bus, count = 0;
> +	struct pci_dev *dev = NULL;
> +	u32 val = 0;
> 
> -	while (1) {
> -		chabox_dev = pci_get_device(PCI_VENDOR_ID_INTEL, 0x208d,
> chabox_dev);
> -		if (!chabox_dev)
> -			break;
> -		if (count == 0)
> -			bus = chabox_dev->bus->number;
> -		if (bus != chabox_dev->bus->number)
> -			break;
> -		count++;
> -	}
> +	dev = pci_get_device(PCI_VENDOR_ID_INTEL, 0x2083, dev);
> +	if (!dev)
> +		return 0;
> 
> -	pci_dev_put(chabox_dev);
> -	return count;
> +	pci_read_config_dword(dev, SKX_CAPID6, &val);
> +	return bitmap_weight((unsigned long *)&val, SKX_CHA_BIT_WIDTH);
>   }
> 
>   void skx_uncore_cpu_init(void)
> --
> 2.7.4