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

[Yi Min Zhao]

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