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block: qcow: Move iovec scatter/gather helpers to qcow_common

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Move scatter_to_iovecs, zero_fill_iovecs and gather_from_iovecs into
qcow_common so they can be shared with the upcoming qcow_async backend.

These helpers treat an iovec array as a flat byte stream and are used by
both read_vectored and write_vectored code paths.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
This commit is contained in:
Anatol Belski
2026-03-20 13:55:23 +01:00
committed by Rob Bradford
parent 7f3dfe2154
commit 81f43f96c3
2 changed files with 99 additions and 92 deletions
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block/src/qcow_sync.rs
+4 -91
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@@ -7,7 +7,7 @@ use std::collections::VecDeque;
use std::fs::File;
use std::os::fd::{AsFd, AsRawFd, BorrowedFd, OwnedFd};
use std::sync::Arc;
use std::{fmt, io, ptr, slice};
use std::{fmt, io};
use vmm_sys_util::eventfd::EventFd;
use vmm_sys_util::write_zeroes::{PunchHole, WriteZeroesAt};
@@ -22,7 +22,9 @@ use crate::qcow::qcow_raw_file::QcowRawFile;
use crate::qcow::{
BackingFile, BackingKind, Error as QcowError, MAX_NESTING_DEPTH, RawFile, parse_qcow,
};
use crate::qcow_common::{pread_exact, pwrite_all};
use crate::qcow_common::{
gather_from_iovecs, pread_exact, pwrite_all, scatter_to_iovecs, zero_fill_iovecs,
};
/// Raw backing file using pread64 on a duplicated fd.
struct RawBacking {
@@ -323,95 +325,6 @@ impl QcowSync {
}
}
/// Copy data into iovecs starting at the given byte offset.
///
/// # Safety
/// Caller must ensure iovecs point to valid, writable memory of sufficient size.
unsafe fn scatter_to_iovecs(iovecs: &[libc::iovec], start: usize, data: &[u8]) {
let mut remaining = data;
let mut pos = 0usize;
for iov in iovecs {
let iov_end = pos + iov.iov_len;
if iov_end <= start || remaining.is_empty() {
pos = iov_end;
continue;
}
let iov_start = start.saturating_sub(pos);
let available = iov.iov_len - iov_start;
let count = min(available, remaining.len());
// SAFETY: iov_base is valid for iov_len bytes per caller contract.
unsafe {
let dst = (iov.iov_base as *mut u8).add(iov_start);
ptr::copy_nonoverlapping(remaining.as_ptr(), dst, count);
}
remaining = &remaining[count..];
if remaining.is_empty() {
break;
}
pos = iov_end;
}
}
/// Zero fill iovecs starting at the given byte offset for the given length.
///
/// # Safety
/// Caller must ensure iovecs point to valid, writable memory of sufficient size.
unsafe fn zero_fill_iovecs(iovecs: &[libc::iovec], start: usize, len: usize) {
let mut remaining = len;
let mut pos = 0usize;
for iov in iovecs {
let iov_end = pos + iov.iov_len;
if iov_end <= start || remaining == 0 {
pos = iov_end;
continue;
}
let iov_start = start.saturating_sub(pos);
let available = iov.iov_len - iov_start;
let count = min(available, remaining);
// SAFETY: iov_base is valid for iov_len bytes per caller contract.
unsafe {
let dst = (iov.iov_base as *mut u8).add(iov_start);
ptr::write_bytes(dst, 0, count);
}
remaining -= count;
if remaining == 0 {
break;
}
pos = iov_end;
}
}
/// Gather bytes from iovecs starting at the given byte offset into a Vec.
///
/// # Safety
/// Caller must ensure iovecs point to valid, readable memory of sufficient size.
unsafe fn gather_from_iovecs(iovecs: &[libc::iovec], start: usize, len: usize) -> Vec<u8> {
let mut result = Vec::with_capacity(len);
let mut remaining = len;
let mut pos = 0usize;
for iov in iovecs {
let iov_end = pos + iov.iov_len;
if iov_end <= start || remaining == 0 {
pos = iov_end;
continue;
}
let iov_start = start.saturating_sub(pos);
let available = iov.iov_len - iov_start;
let count = min(available, remaining);
// SAFETY: iov_base is valid for iov_len bytes per caller contract.
unsafe {
let src = (iov.iov_base as *const u8).add(iov_start);
result.extend_from_slice(slice::from_raw_parts(src, count));
}
remaining -= count;
if remaining == 0 {
break;
}
pos = iov_end;
}
result
}
impl AsyncIo for QcowSync {
fn notifier(&self) -> &EventFd {
&self.eventfd