class VirtioChain

A |VirtioChain| is a linked list of buffer descriptors, read from the a

|VirtioQueue|.

When a chain is read out of a queue, there will be at least one

|VirtioDescriptor|, made availale by calling |NextDescriptor| on the chain.

If more descriptors are available (indicated by the VRING_DESC_F_NEXT flag),

then the subsequent descriptors will be made available further calls to

|NextDescriptor|.

When processing of the chain is completed, it must be returned back to the

driver by calling |Return|. Once a chain has been returned, there must not

be any further interactions with |VirtioDescriptor|s that have been read

from the chain.

If any bytes have been written to the chain, then the |Used| value must be

set the the exact number of bytes that have written to the chain. Reading

bytes from a chain does not impact the |Used| count.

Ex, to iterate over every descriptor in a |VirtioChain|:

void ProcessQueue(VirtioQueue* queue) {

VirtioChain chain;

VirtioDescriptor descriptor;

while (queue->NextChain(

&chain

)) {

// There was a descriptor chain in the queue. It was been read from the

// avail ring and written to |chain|.

while (chain.NextDescriptor(

&descriptor

)) {

// |descriptor| describes a single buffer in the chain.

if (descriptor.writable) {

uint32_t bytes_written =

WriteToDescriptor(descriptor.addr, descriptor.len);

// Increment the |Used| value by the number of bytes written.

*chain.Used() += bytes_written;

} else {

ReadFromDescriptor(descriptor.addr, descriptor.len);

}

}

// Write the chain to the used ring, passing ownership back to the

// driver.

chain.Return();

}

}

}