Re: [Qemu-devel] [PATCH 8/9] spapr_iommu: Introduce page_shift in sPAPRTCETable

[Alexander Graf <agraf@suse.de>]

On 22.05.14 12:46, Alexey Kardashevskiy wrote:
> On 05/22/2014 08:45 PM, Alexander Graf wrote:
>> On 22.05.14 12:24, Alexey Kardashevskiy wrote:
>>> On 05/22/2014 08:09 PM, Alexander Graf wrote:
>>>> On 22.05.14 01:45, Alexey Kardashevskiy wrote:
>>>>> On 05/22/2014 08:11 AM, Alexander Graf wrote:
>>>>>> On 21.05.14 16:21, Alexey Kardashevskiy wrote:
>>>>>>> At the moment only 4K pages are supported by sPAPRTCETable. Since sPAPR
>>>>>>> spec allows other page sizes and we are going to implement them, we need
>>>>>>> page size to be configrable.
>>>>>>>
>>>>>>> This adds @page_shift into sPAPRTCETable and replaces
>>>>>>> SPAPR_TCE_PAGE_SHIFT
>>>>>>> with it whereever it is possible.
>>>>>>>
>>>>>>> This removes SPAPR_TCE_PAGE_MASK as it is no longer used.
>>>>>>>
>>>>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> [...]
>>
>>>>>>> diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
>>>>>>> index fdd4c07..c9850d4 100644
>>>>>>> --- a/hw/ppc/spapr_pci.c
>>>>>>> +++ b/hw/ppc/spapr_pci.c
>>>>>>> @@ -656,6 +656,7 @@ static void spapr_phb_finish_realize(sPAPRPHBState
>>>>>>> *sphb, Error **errp)
>>>>>>>          sPAPRTCETable *tcet;
>>>>>>>            tcet = spapr_tce_new_table(DEVICE(sphb), sphb->dma_liobn,
>>>>>>> +                               SPAPR_TCE_PAGE_SHIFT,
>>>>>>>                                     0x40000000 >> SPAPR_TCE_PAGE_SHIFT);
>>>>>>>          if (!tcet) {
>>>>>>>              error_setg(errp, "Unable to create TCE table for %s",
>>>>>>> diff --git a/hw/ppc/spapr_vio.c b/hw/ppc/spapr_vio.c
>>>>>>> index b84e481..d7e9e6a 100644
>>>>>>> --- a/hw/ppc/spapr_vio.c
>>>>>>> +++ b/hw/ppc/spapr_vio.c
>>>>>>> @@ -457,6 +457,7 @@ static int spapr_vio_busdev_init(DeviceState *qdev)
>>>>>>>          if (pc->rtce_window_size) {
>>>>>>>              uint32_t liobn = SPAPR_VIO_BASE_LIOBN | dev->reg;
>>>>>>>              dev->tcet = spapr_tce_new_table(qdev, liobn,
>>>>>>> +                                        SPAPR_TCE_PAGE_SHIFT,
>>>>>> I don't quite understand who defines what the TCE page size is for a
>>>>>> given
>>>>>> device. Can you try to explain this to me?
>>>>> If it is default window (for PCI) or window for VIO - it is 4K. If it is a
>>>>> dynamic DMA window - page size is a parameter of RTAS call which creates
>>>>> the window.
>>>> Could we change that default size for non-dynamic windows somehow? 4k is
>>>> really fine grained.
>>> No, this is hardcoded in a million places and old distros. Normally it is
>>> maximum 1GB or 2MB table, not too bad. And with DDW support, we can make
>>> the default one really small (64MB?) and lose even less.
>>>
>>> SPAPR:
>>> R1–7.3.31–4. For the Dynamic DMA Windows (DDW) option: The platform must
>>> provide a default DMA window
>>> for each PE, and all of the following must be true:
>>> a. The window is defined by the “ibm,dma-window” property in the OF device
>>> tree.
>>> b. The window is defined with 4 KB I/O pages.
>>> c. The window is located entirely below 4 GB.
>> Cool, would be interesting to see how that affects performance. I like most
>> of this patch set btw, I think it should be good to go in in the next round.
>
> 10Gbit ethernet is ok but 40Gbit seems to suffer from this limitation.

Oh, I was implying that it'd be interesting to see how we fare when we 
do a really small default DMA window to force the guest to use DDW and 
preferably even with very large pages.


Alex