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SubjectRe: [PATCH v2 2/4] xen-pcifront: Align address of flags to size of unsigned long
On Tue, Nov 26, 2019 at 11:02:13AM +0100, Jürgen Groß wrote:
> On 25.11.19 20:43, Fenghua Yu wrote:
> >The address of "flags" is passed to atomic bitops which require the
> >address is aligned to size of unsigned long.
> >
> >Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> ---
> > include/xen/interface/io/pciif.h | 7 +++++-- 1 file changed, 5
> > insertions(+), 2 deletions(-)
> >
> >diff --git a/include/xen/interface/io/pciif.h
> >b/include/xen/interface/io/pciif.h index d9922ae36eb5..639d5fb484a3
> >100644 --- a/include/xen/interface/io/pciif.h +++
> >b/include/xen/interface/io/pciif.h @@ -103,8 +103,11 @@ struct
> >xen_pcie_aer_op {
> > uint32_t devfn;
> > };
> > struct xen_pci_sharedinfo { - /* flags - XEN_PCIF_* */ - uint32_t
> >flags; + /* flags - XEN_PCIF_*. Force alignment for atomic bit
> >operations. */ + union { + uint32_t flags; + unsigned long
> >flags_alignment; + };
> > struct xen_pci_op op; struct xen_pcie_aer_op aer_op;
> > };
> >
>
> NAK.
>
> This is an interface definition for communication between Xen dom0 and
> guests via shared memory. It can't be changed.
>
> BTW: you should Cc: the maintainers for the files you are modifying.
>
>
> Juergen

After spending more time on the patch, actually I find this patch is not
needed because there is no split lock issue on the "flags" field:

The bit offsets in all atomic bitops on the "flags" field are always
constant values. On x86, an atomic bitop is optimized to use a locked byte
instruciton which only operates the byte containing the bit. So the
byte won't across cache line boundaries and there won't be split lock
issue in the atomic bitops

I will remove this patch in the next version of the patch set.

Thanks.

-Fenghua

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