devices: virtio: block: advertise seg_max

The virtio-blk configuration space has a `seg_max` field that lets the
device inform the driver of the maximum number of segments allowed
within a single request.  The Linux virtio block driver assumes that if
the corresponding feature (VIRTIO_BLK_F_SEG_MAX) is not advertised, then
only one segment can be used.

Add a segment limit based on sysconf(_SC_IOV_MAX) to allow the Linux
block stack to make use of multiple segments in a single request, which
will get translated into a single readv/writev call in the crosvm block
device.

BUG=None
TEST=strace crosvm virtio-blk process and note preadv with iov_cnt > 1

Change-Id: Ia14ebebb85daa21e2d43437bb74886f32e6e8187
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1876806
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/sys_util/src/lib.rs b/sys_util/src/lib.rs
index 594c79a..331dbac 100644
--- a/sys_util/src/lib.rs
+++ b/sys_util/src/lib.rs
@@ -80,7 +80,7 @@ use std::ptr;
 
 use libc::{
     c_long, gid_t, kill, pid_t, pipe2, syscall, sysconf, uid_t, waitpid, O_CLOEXEC, SIGKILL,
-    WNOHANG, _SC_PAGESIZE,
+    WNOHANG, _SC_IOV_MAX, _SC_PAGESIZE,
 };
 
 use syscall_defines::linux::LinuxSyscall::SYS_getpid;
@@ -92,6 +92,12 @@ pub fn pagesize() -> usize {
     unsafe { sysconf(_SC_PAGESIZE) as usize }
 }
 
+/// Safe wrapper for `sysconf(_SC_IOV_MAX)`.
+pub fn iov_max() -> usize {
+    // Trivially safe
+    unsafe { sysconf(_SC_IOV_MAX) as usize }
+}
+
 /// Uses the system's page size in bytes to round the given value up to the nearest page boundary.
 #[inline(always)]
 pub fn round_up_to_page_size(v: usize) -> usize {