Re: [Xen-devel] [PATCH] x86/mce: Use MCG_CAP MSR to find out number of banks on AMD

[Borislav Petkov <bp@xxxxxxxxx>]

On Thu, Mar 14, 2013 at 12:11:18PM -0400, Boris Ostrovsky wrote:
> Currently number of error reporting register banks is hardcoded to
> 6 on AMD processors. This may break in virtualized scenarios when
> a hypervisor prefers to report fewer banks that the physical HW
> provides.
> 
> Since number of supported banks is reported in MSR_IA32_MCG_CAP[7:0]
> that's what we should use.

Yes, I definitely like it.

A couple of suggestions below :

> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/mcheck/mce_amd.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce_amd.c 
> b/arch/x86/kernel/cpu/mcheck/mce_amd.c
> index 1ac581f..cb7c739 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce_amd.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce_amd.c
> @@ -33,7 +33,6 @@
>  #include <asm/mce.h>
>  #include <asm/msr.h>
>  
> -#define NR_BANKS          6
>  #define NR_BLOCKS         9
>  #define THRESHOLD_MAX     0xFFF
>  #define INT_TYPE_APIC     0x00020000
> @@ -57,9 +56,9 @@ static const char * const th_names[] = {
>       "execution_unit",
>  };
>  
> -static DEFINE_PER_CPU(struct threshold_bank * [NR_BANKS], threshold_banks);
> +static DEFINE_PER_CPU(struct threshold_bank **, threshold_banks);
>  
> -static unsigned char shared_bank[NR_BANKS] = {
> +static unsigned char shared_bank[MAX_NR_BANKS] = {

This shared_bank thing is a kinda clumsy way of saying that bank 4 is
shared. Great, with this change we're allocating a static array of 32
unsigned chars just to ask whether bank 4 is shared. :-)

I know, I know, this was there before but maybe we could clean it up
properly while at it.

IOW, we probably want to kill that in a pre-patch and replace the test
with:

        /* is this a shared bank */
        if (bank == 4)

The comment should explain why we're testing this way.

For the future, if we get more shared banks, we could introduce a
is_shared_bank() helper but no need to do it just yet.

>       0, 0, 0, 0, 1
>  };
>  
> @@ -214,7 +213,7 @@ void mce_amd_feature_init(struct cpuinfo_x86 *c)
>       unsigned int bank, block;
>       int offset = -1;
>  
> -     for (bank = 0; bank < NR_BANKS; ++bank) {
> +     for (bank = 0; bank < mca_cfg.banks; ++bank) {
>               for (block = 0; block < NR_BLOCKS; ++block) {
>                       if (block == 0)
>                               address = MSR_IA32_MC0_MISC + bank * 4;
> @@ -276,7 +275,7 @@ static void amd_threshold_interrupt(void)
>       mce_setup(&m);
>  
>       /* assume first bank caused it */
> -     for (bank = 0; bank < NR_BANKS; ++bank) {
> +     for (bank = 0; bank < mca_cfg.banks; ++bank) {
>               if (!(per_cpu(bank_map, m.cpu) & (1 << bank)))
>                       continue;
>               for (block = 0; block < NR_BLOCKS; ++block) {
> @@ -467,7 +466,7 @@ static __cpuinit int allocate_threshold_blocks(unsigned 
> int cpu,
>       u32 low, high;
>       int err;
>  
> -     if ((bank >= NR_BANKS) || (block >= NR_BLOCKS))
> +     if ((bank >= mca_cfg.banks) || (block >= NR_BLOCKS))
>               return 0;
>  
>       if (rdmsr_safe_on_cpu(cpu, address, &low, &high))
> @@ -637,7 +636,12 @@ static __cpuinit int threshold_create_device(unsigned 
> int cpu)
>       unsigned int bank;
>       int err = 0;
>  
> -     for (bank = 0; bank < NR_BANKS; ++bank) {
> +     per_cpu(threshold_banks, cpu) = kzalloc(sizeof(struct threshold_bank *)
> +             * mca_cfg.banks, GFP_KERNEL);

per_cpu accesses are not cheap. You should define a local pointer here
and use it instead in all the calls and do the per_cpu assignment only
at the end:

        per_cpu(threshold_banks, cpu) = local_ptr;

> +     if (per_cpu(threshold_banks, cpu) == NULL)
> +             return -ENOMEM;

Which makes this test much more readable too:

        if (!local_ptr)
                return -ENOMEM;


Btw, those threshold_{create,remove}_device are the hotplug callbacks
and the alloc/dealloc looks right but you might want to stress them a
bit by taking cores on- and offline while testing, just in case.

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--

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