Re: [Qemu-devel] [qemu-s390x] [PATCH] s390x/pci: add common fmb

[Yi Min Zhao <zyimin@linux.ibm.com>]

在 2018/10/1 下午5:22, Thomas Huth 写道:
> On 2018-09-29 07:48, Yi Min Zhao wrote:
>> 在 2018/9/19 下午3:53, Thomas Huth 写道:
>>> On 2018-09-19 09:08, Yi Min Zhao wrote:
> [...]
>>>>> diff --git a/hw/s390x/s390-pci-bus.h b/hw/s390x/s390-pci-bus.h
>>>>> index 1f7f9b5814..fdf13a19c0 100644
>>>>> --- a/hw/s390x/s390-pci-bus.h
>>>>> +++ b/hw/s390x/s390-pci-bus.h
>>>>> @@ -286,6 +286,28 @@ typedef struct S390PCIIOMMUTable {
>>>>>         S390PCIIOMMU *iommu[PCI_SLOT_MAX];
>>>>>     } S390PCIIOMMUTable;
>>>>>     +/* Function Measurement Block */
>>>>> +#define DEFAULT_MUI 4000
>>>>> +#define UPDATE_U_BIT 0x1ULL
>>>>> +#define FMBK_MASK 0xfULL
>>>>> +
>>>>> +typedef struct ZpciFmbFmt0 {
>>>>> +    uint64_t dma_rbytes;
>>>>> +    uint64_t dma_wbytes;
>>>>> +} ZpciFmbFmt0;
>>>>> +
>>>>> +typedef struct ZpciFmb {
>>>>> +    uint8_t format;
>>>>> +    uint8_t fmt_ind[3];
>>>>> +    uint32_t sample;
>>>>> +    uint64_t last_update;
>>>>> +    uint64_t ld_ops;
>>>>> +    uint64_t st_ops;
>>>>> +    uint64_t stb_ops;
>>>>> +    uint64_t rpcit_ops;
>>>>> +    ZpciFmbFmt0 fmt0;
>>>>> +} QEMU_PACKED __attribute((__aligned__(8))) ZpciFmb;
>>> All the fields in this struct are naturally aligned already, so I'd
>>> maybe rather drop the QEMU_PACKED and add a
>>> QEMU_BUILD_BUG_MSG(sizeof(ZpciFmb) != xx, ...) statement afterwards.
>> Currently we only implement FMT0. There're other FMTs to be implemented
>> in future.
>> So here there would be a union and we can't give a specific size to
>> QEMU_BUILD_BUG_MSG.
>> Can we use the max size for checking?
> I think you could use this to check the beginning of the struct:
At the beginning of the struct? not after it?
>
> QEMU_BUILD_BUG_MSG(offsetof(ZpciFmb, fmt0) != 48, "padding in ZpciFmb");
I think this could satisfy our requirement.
>
>>>>>     struct S390PCIBusDevice {
>>>>>         DeviceState qdev;
>>>>>         PCIDevice *pdev;
>>>>> @@ -297,6 +319,8 @@ struct S390PCIBusDevice {
>>>>>         uint32_t fid;
>>>>>         bool fid_defined;
>>>>>         uint64_t fmb_addr;
>>>>> +    ZpciFmb fmb;
>>> ... since you embed it here in another struct which does not have any
>>> alignment requirements. This is likely going to cause an error with GCC
>>> 8.1, we've had this problem in the past already:
>>>
>>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=a6e4385dea94850d7b06b0
>> Ah...I didn't test the code with gcc 8+. GCC I used is 7.2.
>> It should get the same warining.
> Nobody reported the warning in the s390-ccw bios until GCC 8 had been
> released, so I assume this is a new warning in GCC 8.
>
>>> Is the __align__(8) required at all? As far as I understand the code,
>>> the struct is not living inside the guest memory, is it? So you could
>>> simply drop the __align__(8).
>>> But if you need it, I think you have to allocate the memory for ZpciFmb
>>> separately (and use a "ZpciFmb *fmb" here instead).
>> I want to copy the entire structure to the guest memory during updating
>> FMB.
>> It's not a good idea to do copy for all members multiple times.
> Sure, but you're doing the updates through address_space_write(), so as
> far as I can see there is currently no need for the
> attribute((__aligned__(8))) here (or did I miss something?). Thus I'd
> like to suggest to simply remove that attribute here.
>
>   Thomas
>
>
Agree to remove it.

-- 
Yi Min