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Re: [PATCH] pvcalls: Document explicitly the padding for all arches


On 22/04/2020 10:20, Jan Beulich wrote:
Even if it was possible to use the sub-structs defined in the header
that way, keep in mind that we also wrote:

         /* dummy member to force sizeof(struct xen_pvcalls_request)
          * to match across archs */
         struct xen_pvcalls_dummy {
             uint8_t dummy[56];
         } dummy;

This has nothing to do with how a consumer may use the structs.

And the spec also clarifies that the size of each specific request is
always 56 bytes.

Sure, and I didn't mean to imply that a consumer would be allowed
to break this requirement. Still something like this

int pvcall_new_socket(struct xen_pvcalls_socket *s) {
     struct xen_pvcalls_request req = {
         .req_id = REQ_ID,
         .cmd = PVCALLS_SOCKET,
         .u.socket = *s,

     return pvcall(&req);

may break.

I think I understand your concern now. So yes I agree this would break 32-bit consumer.

As the padding is at the end of the structure, I think a 32-bit frontend and 64-bit backend (or vice-versa) should currently work without any trouble. The problem would come later if we decide to extend a command.

I will document the padding only for non 32-bit x86 guest and rework the documentation.


Julien Grall



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