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Re: [PATCH 3/6] x86/mem-paging: use guest handle for XENMEM_paging_op_prep

On 20/04/2020 13:35, Jan Beulich wrote:
On 20.04.2020 14:31, Jan Beulich wrote:
On 20.04.2020 14:20, Julien Grall wrote:
On 20/04/2020 13:12, Jan Beulich wrote:
On 20.04.2020 14:08, Julien Grall wrote:
Are the unions plain ones? I could see room for behavior like
the one you describe with transparent unions, albeit still
not quite like you describe it. Getting handle types to be
properly type-checked by the compiler is pretty imperative imo.

It looks like x86 is using structure, but arm is using plain union:

#define ___DEFINE_XEN_GUEST_HANDLE(name, type)                  \
     typedef union { type *p; unsigned long q; }                 \
         __guest_handle_ ## name;                                \
     typedef union { type *p; uint64_aligned_t q; }              \
         __guest_handle_64_ ## name

I don't see how this would make a difference, and hence ...

I will look at introducing a union on Arm.

... how this would help. I must be missing something, or there
must be a very curious bug in only the Arm gcc.

Compiling this (for x86) properly raises two errors:

union u1 { void *p; unsigned long v; };
union u2 { void *p; unsigned long v; };

void test(union u1 u1, union u2 u2) {
        test(u1, u1);
        test(u2, u2);

You are right. It is just me not compiling properly. Sorry for the noise :(


Julien Grall



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