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devices: virtio: add peek operation to queue

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This allows the caller to grab a buffer without committing to using it,
which can be used in the case where two resources (a virtio buffer plus
some other resource) need to be acquired simultaneously.

BUG=None
TEST=build_test.py

Change-Id: Icb61de99db807648ff02c41f95b3128ecce41501
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1904638
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Commit-Queue: Stephen Barber <smbarber@chromium.org>
This commit is contained in:
Daniel Verkamp 2019-11-07 14:18:41 -08:00 committed by Commit Bot
parent aa7f102ef8
commit 279248255f
1 changed files with 16 additions and 5 deletions
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21
devices/src/virtio/queue.rs
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@ -295,8 +295,9 @@ impl Queue {
}
}
/// If a new DescriptorHead is available, returns one and removes it from the queue.
pub fn pop<'a>(&mut self, mem: &'a GuestMemory) -> Option<DescriptorChain<'a>> {
/// Get the first available descriptor chain without removing it from the queue.
/// Call `pop_peeked` to remove the returned descriptor chain from the queue.
pub fn peek<'a>(&mut self, mem: &'a GuestMemory) -> Option<DescriptorChain<'a>> {
if !self.is_valid(mem) {
return None;
}
@ -316,10 +317,20 @@ impl Queue {
// This index is checked below in checked_new.
let descriptor_index: u16 = mem.read_obj_from_addr(desc_idx_addr).unwrap();
let descriptor_chain =
DescriptorChain::checked_new(mem, self.desc_table, queue_size, descriptor_index, 0);
DescriptorChain::checked_new(mem, self.desc_table, queue_size, descriptor_index, 0)
}
/// Remove the first available descriptor chain from the queue.
/// This function should only be called immediately following `peek`.
pub fn pop_peeked(&mut self) {
self.next_avail += Wrapping(1);
}
/// If a new DescriptorHead is available, returns one and removes it from the queue.
pub fn pop<'a>(&mut self, mem: &'a GuestMemory) -> Option<DescriptorChain<'a>> {
let descriptor_chain = self.peek(mem);
if descriptor_chain.is_some() {
self.next_avail += Wrapping(1);
self.pop_peeked();
}
descriptor_chain
}