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virtio_ring: document alignment requirements
Host needs to know vring element alignment requirements: simply doing alignof on structures doesn't work reliably: on some platforms gcc has alignof(uint32_t) == 2. Add macros for alignment as specified in virtio 1.0 cs01, export them to userspace as well. Acked-by: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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@ -101,6 +101,13 @@ struct vring {
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struct vring_used *used;
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};
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/* Alignment requirements for vring elements.
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* When using pre-virtio 1.0 layout, these fall out naturally.
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*/
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#define VRING_AVAIL_ALIGN_SIZE 2
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#define VRING_USED_ALIGN_SIZE 4
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#define VRING_DESC_ALIGN_SIZE 16
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/* The standard layout for the ring is a continuous chunk of memory which looks
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* like this. We assume num is a power of 2.
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*
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