Re: [PATCH] x86: Report Intel platform_id in /proc/cpuinfo
From: Henrique de Moraes Holschuh
Date: Thu Jun 02 2016 - 14:26:52 EST
On Tue, 31 May 2016, Andi Kleen wrote:
> We have a need to distinguish systems based on their platform ID.
> For example this is useful to distinguish systems with L4 cache
> versus ones without.
> There is a 5 bit identifier (also called processor flags) in
There is a 3 bit identifier...
> the IA32_PLATFORM_ID MSR that can give a more fine grained
> identification of the CPU than just the model number/stepping.
> IA32_PLATFORM_ID is architectural.
> The processor flags are already used in the microcode driver.
> The MSR can be also accessed through /dev/cpu/*/msr, but that
> requires root and is awkward.
> This patch just exports the value retrieved by the microcode
> driver in /proc/cpuinfo. If the microcode driver is disabled
> it won't be shown, but that seems reasonable.
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index ee81c54..6244a88 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -812,6 +812,7 @@ static int collect_cpu_info(int cpu_num, struct cpu_signature *csig)
> /* get processor flags from MSR 0x17 */
> rdmsr(MSR_IA32_PLATFORM_ID, val, val);
> csig->pf = 1 << ((val >> 18) & 7);
> + cpu_data(cpu_num).platform_id = (val >> 18) & 7;
See below. It might be better to have "cpu_data(cpu_num).platform_id =
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 18ca99f..1c4e4f5 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -76,6 +76,8 @@ static int show_cpuinfo(struct seq_file *m, void *v)
> seq_puts(m, "stepping\t: unknown\n");
> if (c->microcode)
> seq_printf(m, "microcode\t: 0x%x\n", c->microcode);
> + if (c->platform_id)
> + seq_printf(m, "platform_id\t: %d\n", c->platform_id);
platform_id can meaningfully be zero on Intel, and in fact it often is (just
look for output from the microcode driver that logs pf=0x1). In those
processors, (MSR(17) >> 18 & 7) is zero, and csig->pf is 1.
May I humbly suggest using the mask "(1 << value)" notation for platform_id
as used by the microcode driver? We already do it for csig->pf, and it has
the advantage that mask notation will never be zero (so, the field is always
non-zero where relevant).
Alterntively, the patch could be changed to always print platform_id on
Intel processors, instead of just printing it out when it is non-zero.
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