Re: [Xen-devel] [PATCH] x86: Use deep C states for off-lined CPUs

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

I noticed the following comments when using mwait based idle:
-------------------------------------------------------------------------
        while ( 1 )
        {
            /*
             * 1. The CLFLUSH is a workaround for erratum AAI65 for
             * the Xeon 7400 series.
             * 2. The WBINVD is insufficient due to the spurious-wakeup
             * case where we return around the loop.
             * 3. Unlike wbinvd, clflush is a light weight but not serializing
             * instruction, hence memory fence is necessary to make sure all
             * load/store visible before flush cache line.
             */
            mb();
            clflush(mwait_ptr);
            __monitor(mwait_ptr, 0, 0);
            mb();
            __mwait(cx->address, 0);
        }
    }
-------------------------------------------------------------------------
Your patch should follow it too.

best regards
yang


> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of Boris Ostrovsky
> Sent: Wednesday, February 29, 2012 6:09 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: boris.ostrovsky@xxxxxxx
> Subject: [Xen-devel] [PATCH] x86: Use deep C states for off-lined CPUs
> 
> # HG changeset patch
> # User Boris Ostrovsky <boris.ostrovsky@xxxxxxx> # Date 1330466573 -3600 #
> Node ID 9e5991ad9c85b5176ce269001e7957e8805dd93c
> # Parent  a7bacdc5449a2f7bb9c35b2a1334b463fe9f29a9
> x86: Use deep C states for off-lined CPUs
> 
> Currently when a core is taken off-line it is placed in C1 state (unless
> MONITOR/MWAIT is used). This patch allows a core to go to deeper C states
> resulting in significantly higher power savings.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxx>
> 
> diff -r a7bacdc5449a -r 9e5991ad9c85 xen/arch/x86/acpi/cpu_idle.c
> --- a/xen/arch/x86/acpi/cpu_idle.c    Mon Feb 27 17:05:18 2012 +0000
> +++ b/xen/arch/x86/acpi/cpu_idle.c    Tue Feb 28 23:02:53 2012 +0100
> @@ -573,10 +573,10 @@ static void acpi_dead_idle(void)
>      if ( (cx = &power->states[power->count-1]) == NULL )
>          goto default_halt;
> 
> -    mwait_ptr = (void *)&mwait_wakeup(smp_processor_id());
> -
>      if ( cx->entry_method == ACPI_CSTATE_EM_FFH )
>      {
> +        mwait_ptr = (void *)&mwait_wakeup(smp_processor_id());
> +
>          /*
>           * Cache must be flushed as the last operation before sleeping.
>           * Otherwise, CPU may still hold dirty data, breaking cache 
> coherency,
> @@ -601,6 +601,20 @@ static void acpi_dead_idle(void)
>              mb();
>              __mwait(cx->address, 0);
>          }
> +    }
> +    else if ( cx->entry_method == ACPI_CSTATE_EM_SYSIO )
> +    {
> +        /* Avoid references to shared data after the cache flush */
> +        u32 address = cx->address;
> +        u32 pmtmr_ioport_local = pmtmr_ioport;
> +
> +        wbinvd();
> +
> +        while ( 1 )
> +        {
> +            inb(address);
> +            inl(pmtmr_ioport_local);
> +        }
>      }
> 
>  default_halt:
> 
> 
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