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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

> 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).

> --- 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?

Jan

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