docs: add log levels to advanced_usage
Add details on how to change log levels in crosvm.
BUG=N/A
TEST=mdbook build
Change-Id: I2ded0d14c5df9bf2032d8e4fb0896882c754fda5
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4956179
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Commit-Queue: Elie Kheirallah <khei@google.com>
diff --git a/docs/book/src/running_crosvm/advanced_usage.md b/docs/book/src/running_crosvm/advanced_usage.md
index 62401cc..9e06fa4 100644
--- a/docs/book/src/running_crosvm/advanced_usage.md
+++ b/docs/book/src/running_crosvm/advanced_usage.md
@@ -2,6 +2,39 @@
To see the usage information for your version of crosvm, run `crosvm` or `crosvm run --help`.
+## Specify log levels
+
+To change the log levels printed while running crosvm:
+
+```sh
+crosvm --log-level=LEVEL run
+```
+
+Ex:
+
+```sh
+crosvm --log-level=debug run
+```
+
+To change the log levels printed for a specific module:
+
+```sh
+crosvm --log-level=devices::usb::xhci=LEVEL run
+```
+
+Those can be combined to print different log levels for modules and for crosvm:
+
+```sh
+crosvm --log-level=devices::usb::xhci=LEVEL1,LEVEL2 run
+```
+
+Where LEVEL1 will be applied to the module "devices::usb::xhci" and LEVEL2 will be applied to the
+rest of crosvm.
+
+Available LEVELs: off, error, warn, info (default), debug, trace (only available in debug builds).
+
+Note: Logs will print all logs of the same or lower level. Ex: info will print error + warn + info.
+
## Boot a Kernel
To run a very basic VM with just a kernel and default devices:
