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Re: [Xen-devel] [PATCH 5/5] xen: Add V4V implementation - padding question



On 06/11/2013 01:40 PM, Ross Philipson wrote:
On 06/11/2013 01:25 PM, Tim Deegan wrote:
At 13:10 -0400 on 11 Jun (1370956242), Ross Philipson wrote:

...
+struct v4v_info
+{
+ uint64_t ring_magic;
+ uint64_t data_magic;
+ evtchn_port_t evtchn;

Missing padding at the end?


ack.


At one point during the review of an earlier version of the V4V patch
set, Jan requested that this pad be added to the v4v_info struct. I
understand the padding in all the other structs but I don't understand
this one. This struct is not included in any other structs and is not
followed by any data.

64-bit Xen would see this as a 24-byte struct, even without explicit
padding. That would surprise a 32-bit guest that allocated what it saw
as a 20-byte struct for Xen to copy into.

Ah yes, of course. Thanks for the quick response.

Ross

I guess that means that this struct is unhappy then...

typedef struct v4vtables_rule
{
    v4v_addr_t src;  -- 8b
    v4v_addr_t dst;  -- 8b
    uint32_t accept; -- 4b
} v4vtables_rule_t;




Tim.


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