Re: [Xen-devel] [PATCH] x86/hyperv: change hv_tlb_flush_ex to fix clang build

[Wei Liu <wl@xxxxxxx>]

On Mon, Dec 23, 2019 at 10:53:29AM +0100, Jan Beulich wrote:
> On 22.12.2019 00:50, Wei Liu wrote:
> > Clang complains:
> > 
> > In file included from synic.c:15:
> > /builds/xen-project/xen/xen/include/asm/guest/hyperv-tlfs.h:900:18: error: 
> > field 'hv_vp_set' with variable sized type 'struct hv_vpset' not at the end 
> > of a struct or class is a GNU extension 
> > [-Werror,-Wgnu-variable-sized-type-not-at-end]
> >         struct hv_vpset hv_vp_set;
> >                         ^
> 
> I'm surprised gcc does not complain.

Me too.

> 
> > 1 error generated.
> > /builds/xen-project/xen/xen/Rules.mk:198: recipe for target 'synic.o' failed
> > make[6]: *** [synic.o] Error 1
> > 
> > Drop the last variable size array from hv_tlb_flush_ex for now to fix
> > clang builds.
> 
> Why "for now"? The field is illegal there, and I can't see a way to
> make it legal. To know its actual place in memory you need to know
> the number of elements in bank_contents[], i.e. having the field
> if the compiler erroneously accepted it would mean wrong code to
> get generated (silently).
> 

OK. I can drop "for now".

> > --- a/xen/include/asm-x86/guest/hyperv-tlfs.h
> > +++ b/xen/include/asm-x86/guest/hyperv-tlfs.h
> > @@ -893,12 +893,11 @@ struct hv_tlb_flush {
> >     u64 gva_list[];
> >  } __packed;
> >  
> > -/* HvFlushVirtualAddressSpaceEx, HvFlushVirtualAddressListEx hypercalls */
> > +/* HvFlushVirtualAddressSpaceEx hypercall */
> >  struct hv_tlb_flush_ex {
> >     u64 address_space;
> >     u64 flags;
> >     struct hv_vpset hv_vp_set;
> > -   u64 gva_list[];
> 
> Rather than outright removing the line, keep it as a comment?

That's fine by me.

Wei.

> 
> Jan

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