Re: [Qemu-devel] [Qemu-ppc] [PATCH 11/13] pseries: Fixes and enhancements to L1 cache properties

[Alexander Graf]

On 19.12.2012, at 05:34, David Gibson wrote:

> On Tue, Dec 18, 2012 at 12:49:02AM +0100, Alexander Graf wrote:
>> 
>> On 18.12.2012, at 00:00, David Gibson wrote:
>> 
>>> On Mon, Dec 17, 2012 at 11:10:12AM +0100, Alexander Graf wrote:
>>>> 
>>>> 
>>>> On 17.12.2012, at 03:32, David Gibson <address@hidden> wrote:
>>>> 
>>>>> On Thu, Dec 13, 2012 at 01:50:25PM +0100, Alexander Graf wrote:
>>>>>> 
>>>>>> On 04.12.2012, at 03:42, David Gibson wrote:
>>>>>> 
>>>>>>> PAPR requires that the device tree's CPU nodes have several properties
>>>>>>> with information about the L1 cache.  We created two of these
>>>>>>> properties, but with incorrect names - "[id]cache-block-size" instead
>>>>>>> of "[id]-cache-block-size" (note the extra hyphen).
>>>>>>> 
>>>>>>> We were also missing some of the required cache properties.  This
>>>>>>> patch adds the [id]-cache-line-size properties (which have the same
>>>>>>> values as the block size properties in all current cases).  We also
>>>>>>> add the [id]-cache-size properties.  The latter requires some extra
>>>>>>> infrastructure in the general target-ppc code to (optionally) set the
>>>>>>> cache sizes for various CPUs.  We obtain the published values either
>>>>>>> from there, or from the host when KVM is in use.
>>>>>>> 
>>>>>>> Signed-off-by: David Gibson <address@hidden>
>>>>>>> ---
>>>>>>> hw/spapr.c                  |   20 ++++++++++++++++++--
>>>>>>> target-ppc/cpu.h            |    1 +
>>>>>>> target-ppc/kvm.c            |   10 ++++++++++
>>>>>>> target-ppc/kvm_ppc.h        |   12 ++++++++++++
>>>>>>> target-ppc/translate_init.c |    4 ++++
>>>>>>> 5 files changed, 45 insertions(+), 2 deletions(-)
>>>>>>> 
>>>>>>> diff --git a/hw/spapr.c b/hw/spapr.c
>>>>>>> index d23aa9d..3bacf2f 100644
>>>>>>> --- a/hw/spapr.c
>>>>>>> +++ b/hw/spapr.c
>>>>>>> @@ -315,6 +315,10 @@ static void *spapr_create_fdt_skel(const char 
>>>>>>> *cpu_model,
>>>>>>>                         0xffffffff, 0xffffffff};
>>>>>>>      uint32_t tbfreq = kvm_enabled() ? kvmppc_get_tbfreq() : 
>>>>>>> TIMEBASE_FREQ;
>>>>>>>      uint32_t cpufreq = kvm_enabled() ? kvmppc_get_clockfreq() : 
>>>>>>> 1000000000;
>>>>>>> +        int dcache_size = kvm_enabled() ? kvmppc_get_dcache_size()
>>>>>>> +            : env->l1_dcache_size;
>>>>>>> +        int icache_size = kvm_enabled() ? kvmppc_get_icache_size()
>>>>>>> +            : env->l1_icache_size;
>>>>>> 
>>>>>> By default with KVM we use -cpu host, right? So we already should
>>>>>> get the correct cache sizes for the CPU you're on.
>>>>> 
>>>>> Um.. sort of.  The first problem with that is that I only just added
>>>>> the cache size information to qemu, so only a few CPUs currently
>>>>> populate that information.  Using the host info means we can get the
>>>>> right information even for CPUs that don't yet have cache info in
>>>>> qemu.
>>>>> 
>>>>>> Imagine we would support the compatibility feature where you could
>>>>>> run with -cpu POWER6 on a POWER7 machine. Would exposing the POWER6
>>>>>> cache size rather than the host's make any real difference to the
>>>>>> guest? Or would it work nevertheless?
>>>>> 
>>>>> The second problem is that there may be circumstances where the
>>>>> cache size is altered from the normal size for the cpu.  Running in
>>>>> POWER6 compat mode
>>>> 
>>>> Well, either we want to be compatible or we don't :). If we run with
>>>> -cpu POWER6 we want to generate the same dt as we did on a POWER6
>>>> system itself.
>>> 
>>> Hrm.  Ok.
>>> 
>>> So, the remaining difficulty I have with that is that for -cpu HOST we
>>> should still take the cache sizes from the host, but that can't easily
>>> be done because they're only stored in the env, not the cpu_def.
>> 
>> Can we set a bit somewhere that allows us to do a sanity check
>> later? After all, the values coming from the host and the values in
>> the populated env really should just be identical for -cpu
>> host. Every time they're not, it's simply a bug that needs to be
>> reported.
> 
> That works.  Although it's not obvious where to put the check and
> fixup.  kvmppc_fixup_cpu() seems like the obvious place, but that's no
> good because it's called before the per-cpu-type init function, which
> is what populates the expected cachesize values.

That's a real shame. Any reason we don't run it after the init function? Fixup 
indicates that it fixes things up after they happened, not before :).


Alex