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 06/12/2018 08:54, Jan Beulich wrote:
>>>> On 05.12.18 at 18:05, <andrew.cooper3@xxxxxxxxxx> wrote:
>> 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.
> If you look back at when it was introduced, the main goal was
> for it to never be too low. Any improvements to its calculation
> are welcome, provided they maintain that guarantee. To high
> a socket count is imo still better than a hard-coded one.

Even for the extra 2k of memory it will waste?

~Andrew

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