Re: [PATCH 1/1] qemu_command: tidy up qemuBuildHostdevCommandLine loop

Ján Tomko <jtomko@xxxxxxxxxx> · Mon, 7 Oct 2019 15:10:29 +0200

On Wed, Oct 02, 2019 at 09:37:38AM -0300, Daniel Henrique Barboza wrote:
The current 'for' loop with 5 consecutive 'ifs' inside
qemuBuildHostdevCommandLine can be a bit smarter:

- all 5 'ifs' fails if hostdev->mode is not equal to
VIR_DOMAIN_HOSTDEV_MODE_SUBSYS. This check can be moved to the
start of the loop, failing to the next element immediately
in case it fails;

- all 5 'ifs' checks for a specific subsys->type to build the proper
command line argument (virHostdevIsSCSIDevice and virHostdevIsMdevDevice
do that but within a helper). Problem is that the code will keep
checking for matches even if one was already found, and there is
no way a hostdev will fit more than one 'if' (i.e. a hostdev can't
have 2+ different types). This means that a SUBSYS_TYPE_USB will
create its command line argument in the first 'if', then all other
conditionals will surely fail but will end up being checked anyway.
This can be avoided by adding 'continue' statements inside the first
4 conditionals.

Signed-off-by: Daniel Henrique Barboza <danielhb413@xxxxxxxxx>
---

Arguably safe for freeze since it's trivial, but not a deal
breaker if postponed. I am fine with both.


Whether it's applicable for freeze is a better question - postponing
refactors that do not have any functional impact does not have a benefit
for the users of that particular release and risks introducing a bug
in case we misjudged the 'no functional impact' part.


src/qemu/qemu_command.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 8c979fdf65..0357481dd1 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -5471,20 +5471,23 @@ qemuBuildHostdevCommandLine(virCommandPtr cmd,
        virDomainHostdevSubsysPtr subsys = &hostdev->source.subsys;
        VIR_AUTOFREE(char *) devstr = NULL;

+        if (hostdev->mode != VIR_DOMAIN_HOSTDEV_MODE_SUBSYS)
+            continue;
+

Moving the mode == SUBSYS check here does make it more readable (and
makes sense, given that we fill the subsys pointer above regardless of
the actual mode)

        /* USB */
-        if (hostdev->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
-            subsys->type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_USB) {
+        if (subsys->type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_USB) {

            virCommandAddArg(cmd, "-device");
            if (!(devstr =
                  qemuBuildUSBHostdevDevStr(def, hostdev, qemuCaps)))
                return -1;
            virCommandAddArg(cmd, devstr);
+
+            continue;

IMO using a switch here would be even better.

Jano

        }

        /* PCI */
-        if (hostdev->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
-            subsys->type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_PCI) {
+        if (subsys->type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_PCI) {
            int backend = subsys->u.pci.backend;

            if (backend == VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO) {
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