Re: [Xen-devel] [PATCH 6/9] x86/amd: Allocate resources to cope with LS_CFG being per-core on Fam17h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/12/2018 16:57, Jan Beulich wrote:
>>>> On 03.12.18 at 17:18, <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/cpu/amd.c
>> +++ b/xen/arch/x86/cpu/amd.c
>> @@ -419,6 +419,97 @@ static void __init noinline amd_probe_legacy_ssbd(void)
>>  }
>>  
>>  /*
>> + * This is all a gross hack, but Xen really doesn't have flexible-enough
>> + * per-cpu infrastructure to do it properly.  For Zen(v1) with SMT active,
>> + * MSR_AMD64_LS_CFG is per-core rather than per-thread, so we need a 
>> per-core
>> + * spinlock to synchronise updates of the MSR.
>> + *
>> + * We can't use per-cpu state because taking one CPU offline would free 
>> state
>> + * under the feet of another.  Ideally, we'd allocate memory on the AP boot
>> + * path, but by the time the sibling information is calculated sufficiently
>> + * for us to locate the per-core state, it's too late to fail the AP boot.
>> + *
>> + * We also can't afford to end up in a heterogeneous scenario with some CPUs
>> + * unable to safely use LS_CFG.
>> + *
>> + * Therefore, we have to allocate for the worse-case scenario, which is
>> + * believed to be 4 sockets.  Any allocation failure cause us to turn LS_CFG
>> + * off, as this is fractionally better than failing to boot.
>> + */
>> +static struct ssbd_ls_cfg {
>> +    spinlock_t lock;
>> +    unsigned int disable_count;
>> +} *ssbd_ls_cfg[4];
> Same question as to Brian for his original code: Instead of the
> hard-coding of 4, can't you use nr_sockets here?
> smp_prepare_cpus() runs before pre-SMP initcalls after all.

nr_sockets has zero connection with reality as far as I can tell.

On this particular box it reports 6 when the correct answer is 2.  I've
got some Intel boxes where nr_sockets reports 15 and the correct answer
is 4.

~Andrew

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