git: 9a685c09f06a - main - EC2: Remove old broken_txfifo workaround
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Date: Wed, 14 May 2025 23:54:14 UTC
The branch main has been updated by cperciva:
URL: https://cgit.FreeBSD.org/src/commit/?id=9a685c09f06a55b18589d75f9307563d84a17fa9
commit 9a685c09f06a55b18589d75f9307563d84a17fa9
Author: Colin Percival <cperciva@FreeBSD.org>
AuthorDate: 2025-05-14 23:36:26 +0000
Commit: Colin Percival <cperciva@FreeBSD.org>
CommitDate: 2025-05-14 23:54:04 +0000
EC2: Remove old broken_txfifo workaround
Early versions of Xen, including those used in the early days of EC2,
had a bug in their UART emulation whereby the TX FIFO wouldn't send the
expected interrupt when emptying; as a result, FreeBSD would write 16
characters to the serial console and then stop because we thought the
FIFO was forever full.
In 2013 (1c60b24baa50) I added a loader tunable "hw.broken_txfifo"
which spinwaits for the FIFO TX rather than relying on the interrupt,
and enabled this in loader.conf in EC2 images.
A decade later, this workaround is almost certainly no longer needed in
EC2 -- most instances don't run Xen, and the bug was long since fixed
in Xen anyway -- but we've been holding on to the workaround "just in
case". Unfortunately, the spinwait behaviour is causing latency spikes
and triggering warnings from the ena(4) driver.
This commit removes the hw.broken_txfifo setting from loader.conf in
EC2 images, but leaves the loader tunable and associated code, since it
has been necessary in some other environments. (It seems that the TX
FIFO missing-interrupts bug has been independently written at least
three times!)
MFC after: 1 minute
Sponsored by: Amazon
---
release/tools/ec2.conf | 5 -----
1 file changed, 5 deletions(-)
diff --git a/release/tools/ec2.conf b/release/tools/ec2.conf
index d6cb85959183..ef7a603efea9 100644
--- a/release/tools/ec2.conf
+++ b/release/tools/ec2.conf
@@ -65,11 +65,6 @@ ec2_common() {
# screenshot") which was introduced in 2016.
echo 'boot_multicons="YES"' >> ${DESTDIR}/boot/loader.conf
- # Some older EC2 hardware used a version of Xen with a bug in its
- # emulated serial port. It is not clear if EC2 still has any such
- # nodes, but apply the workaround just in case.
- echo 'hw.broken_txfifo="1"' >> ${DESTDIR}/boot/loader.conf
-
# Graviton 1 through Graviton 4 have a bug in their ACPI where they
# mark the PL061's pins as needing to be configured in PullUp mode
# (in fact the PL061 has no pullup/pulldown resistors). Graviton 1