xen/drivers/char/ns16550.c | 47 +++++++++++++++++++++++++++++++++++++- xen/include/xen/pci_ids.h | 2 ++ 2 files changed, 48 insertions(+), 1 deletion(-)
Amazon EC2 bare metal instances have no UART at the legacy I/O port
0x3f8. Their only serial port is a 16550-compatible PCI device (vendor
0x1d0f, device 0x8250) with its registers in the MMIO space of BAR 0.
Today Xen has no console on these systems unless the command line
names that device, and "com1=...,pci" cannot find it, because
uart_config[] doesn't have it.
Add the device to uart_config[].
Additionally, when the port that com1 describes is not present, scan PCI
for a known UART before giving up. This way the same command line works on
systems with and without a legacy UART, and a machine-specific "pci"
option is not needed. The fallback does not run when the command line
gave an I/O base, or when it already asked for a scan with "pci" or
"amt", so explicit config keeps its current meaning. It is
for com1 only: for com2, pci_uart_config() skips the first port it
finds, so it can never match a single-port device.
When the scan finds nothing, pci_uart_config() puts back the original
base, and check_existence() does not test MMIO addresses.
On a system with a legacy UART, check_existence() passes and nothing
changes.
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Benjamin Leggett <benjamin@edera.io>
---
xen/drivers/char/ns16550.c | 47 +++++++++++++++++++++++++++++++++++++-
xen/include/xen/pci_ids.h | 2 ++
2 files changed, 48 insertions(+), 1 deletion(-)
diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 120ac09d23..d5403598d2 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -64,8 +64,10 @@ static struct ns16550 {
bool intr_works;
bool force_polling;
bool dw_usr_bsy;
+ bool io_base_set; /* if =1, io_base came from the command line */
#ifdef NS16550_PCI
/* PCI card parameters. */
+ bool pci_scanned; /* if =1, pci_uart_config() already ran */
bool pb_bdf_enable; /* if =1, pb-bdf effective, port behind bridge */
bool ps_bdf_enable; /* if =1, ps_bdf effective, port on pci card */
pci_sbdf_t pci_bridge;
@@ -98,6 +100,7 @@ struct ns16550_config {
param_intel_lpss,
param_wch_ch382,
param_asix,
+ param_amazon,
} param;
};
@@ -909,6 +912,13 @@ static const struct ns16550_config_param __initconst uart_param[] = {
.bar0 = true,
.max_ports = 1,
},
+ [param_amazon] = {
+ .reg_width = 1,
+ .lsr_mask = UART_LSR_THRE,
+ .bar0 = true,
+ .mmio = true,
+ .max_ports = 1,
+ },
};
static const struct ns16550_config __initconst uart_config[] =
@@ -1255,6 +1265,12 @@ static const struct ns16550_config __initconst uart_config[] =
.dev_id = 0x9910,
.param = param_asix
},
+ /* Amazon EC2 bare metal UART, the only serial port on those systems */
+ {
+ .vendor_id = PCI_VENDOR_ID_AMAZON,
+ .dev_id = 0x8250,
+ .param = param_amazon
+ },
};
static int __init
@@ -1263,6 +1279,8 @@ pci_uart_config(struct ns16550 *uart, bool skip_amt, unsigned int idx)
u64 orig_base = uart->io_base;
unsigned int b, d, f, nextf, i;
+ uart->pci_scanned = true;
+
/* NB. Start at bus 1 to avoid AMT: a plug-in card cannot be on bus 0. */
for ( b = skip_amt ? 1 : 0; b < 0x100; b++ )
{
@@ -1618,6 +1636,7 @@ static bool __init parse_positional(struct ns16550 *uart, char **str)
#endif
{
uart->io_base = simple_strtoull(conf, &conf, 0);
+ uart->io_base_set = true;
}
}
@@ -1693,6 +1712,7 @@ static bool __init parse_namevalue_pairs(char *str, struct ns16550 *uart)
break;
}
uart->io_base = simple_strtoull(param_value, NULL, 0);
+ uart->io_base_set = true;
break;
case irq:
@@ -1805,7 +1825,32 @@ static void __init ns16550_parse_port_config(
if ( uart->io_base == 0 )
PARSE_ERR("I/O base address must be specified.");
if ( !check_existence(uart) )
- PARSE_ERR("16550-compatible serial UART not present");
+ {
+ bool present = false;
+
+#ifdef NS16550_PCI
+ /*
+ * Some systems, EC2 bare metal among them, have no legacy UART and
+ * carry their only serial port on PCI. Look for one before giving up,
+ * unless the command line named a base or already asked for a scan.
+ * com1 only: for com2 the scan skips the first port it finds, so it
+ * can never match a single-port device.
+ */
+ if ( uart == ns16550_com && !uart->io_base_set && !uart->pci_scanned )
+ {
+ pci_uart_config(uart, 1 /* skip AMT */, uart - ns16550_com);
+ /*
+ * A scan that matched nothing puts back the base we just rejected,
+ * and check_existence() passes MMIO addresses through untested, so
+ * ps_bdf_enable is what says a device was found.
+ */
+ present = uart->ps_bdf_enable && check_existence(uart);
+ }
+#endif
+
+ if ( !present )
+ PARSE_ERR("16550-compatible serial UART not present");
+ }
/* Register with generic serial driver. */
serial_register_uart(uart - ns16550_com, &ns16550_driver, uart);
diff --git a/xen/include/xen/pci_ids.h b/xen/include/xen/pci_ids.h
index fd424ef55d..a17c88dcf7 100644
--- a/xen/include/xen/pci_ids.h
+++ b/xen/include/xen/pci_ids.h
@@ -17,6 +17,8 @@
#define PCI_VENDOR_ID_WCHIC 0x1c00
+#define PCI_VENDOR_ID_AMAZON 0x1d0f
+
#define PCI_VENDOR_ID_INTEL 0x8086
#endif /* XEN_PCI_IDS_H */
--
2.55.0On 22.09.2026 20:43, Benjamin Leggett wrote:
> Amazon EC2 bare metal instances have no UART at the legacy I/O port
> 0x3f8. Their only serial port is a 16550-compatible PCI device (vendor
> 0x1d0f, device 0x8250) with its registers in the MMIO space of BAR 0.
> Today Xen has no console on these systems unless the command line
> names that device, and "com1=...,pci" cannot find it, because
> uart_config[] doesn't have it.
>
> Add the device to uart_config[].
Is there a spec that you could point to here?
> Additionally, when the port that com1 describes is not present, scan PCI
> for a known UART before giving up. This way the same command line works on
> systems with and without a legacy UART, and a machine-specific "pci"
> option is not needed. The fallback does not run when the command line
> gave an I/O base, or when it already asked for a scan with "pci" or
> "amt", so explicit config keeps its current meaning. It is
> for com1 only: for com2, pci_uart_config() skips the first port it
> finds, so it can never match a single-port device.
This needs to be split to a separate patch, and not only because right
now you're doing two entirely unrelated things in a single change. The
addition of the Amazon device is likely uncontroversial, so presumably
can go in quickly. Doing a scan when none was asked for, otoh, is
potentially problematic: What if there's an issue during scanning? With
not having any output set up yet, we couldn't even indicate the problem,
and a possible crash would also go entirely silently.
> When the scan finds nothing, pci_uart_config() puts back the original
> base, and check_existence() does not test MMIO addresses.
>
> On a system with a legacy UART, check_existence() passes and nothing
> changes.
This isn't really true, is it? You check ...
> @@ -1805,7 +1825,32 @@ static void __init ns16550_parse_port_config(
> if ( uart->io_base == 0 )
> PARSE_ERR("I/O base address must be specified.");
> if ( !check_existence(uart) )
> - PARSE_ERR("16550-compatible serial UART not present");
> + {
> + bool present = false;
> +
> +#ifdef NS16550_PCI
> + /*
> + * Some systems, EC2 bare metal among them, have no legacy UART and
> + * carry their only serial port on PCI. Look for one before giving up,
> + * unless the command line named a base or already asked for a scan.
> + * com1 only: for com2 the scan skips the first port it finds, so it
> + * can never match a single-port device.
> + */
> + if ( uart == ns16550_com && !uart->io_base_set && !uart->pci_scanned )
... ->io_base_set here, which can only be true when a command line option
provided the base address. A cmdline option like "com1=115200" doesn't,
and instead the value set by (on x86) __start_xen() is used. If that isn't
the correct address to use, check_existence() is (hopefully) going to fail.
(For example, I have a system where firmware mixes up COM1 and COM2
settings, which you may only notice after having played with things for a
while. In such a case it doesn't help if unexpected PCI bus scanning gets
in the way.)
Jan
Jan Beulich <jbeulich@suse.com> writes: > Is there a spec that you could point to here? Unfortunately not really, just an upstream Linux commit from 2017 (<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3bfd1300abfe3adb18e84a89d97a0e82a22124bb> ), and the fact that I’ve tested it on EC2 bare metal boxes. The bare metal boxes are being phased out, but people still use them. > This needs to be split to a separate patch, and not only because right > now you’re doing two entirely unrelated things in a single change. The > addition of the Amazon device is likely uncontroversial, so presumably > can go in quickly. Yes, agreed. I will split and separately submit them. > Doing a scan when none was asked for, otoh, is > potentially problematic: What if there’s an issue during scanning? With > not having any output set up yet, we couldn’t even indicate the problem, > and a possible crash would also go entirely silently. Yep, thanks. I will see if I can address these with a different version of the autoscan patch as a followup.
Amazon EC2 bare metal instances have no UART at the legacy I/O port
0x3f8. Their only serial port is a 16550-compatible PCI device (vendor
0x1d0f, device 0x8250) with its registers in the MMIO space of BAR 0.
pci_uart_config() uses the default parameters for a device that is not
in uart_config[], and those only accept an I/O BAR, so "com1=...,pci"
cannot find this device. Add it, with the same layout that Linux uses
(commit 3bfd1300abfe ("serial: 8250_pci: Add Amazon PCI serial device
ID")): one port at the start of BAR 0, 1-byte registers, and the
default 1.8432MHz clock.
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Benjamin Leggett <benjamin@edera.io>
---
xen/drivers/char/ns16550.c | 14 ++++++++++++++
xen/include/xen/pci_ids.h | 2 ++
2 files changed, 16 insertions(+)
diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 120ac09d23..593d208483 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -98,6 +98,7 @@ struct ns16550_config {
param_intel_lpss,
param_wch_ch382,
param_asix,
+ param_amazon,
} param;
};
@@ -909,6 +910,13 @@ static const struct ns16550_config_param __initconst uart_param[] = {
.bar0 = true,
.max_ports = 1,
},
+ [param_amazon] = {
+ .reg_width = 1,
+ .lsr_mask = UART_LSR_THRE,
+ .bar0 = true,
+ .mmio = true,
+ .max_ports = 1,
+ },
};
static const struct ns16550_config __initconst uart_config[] =
@@ -1255,6 +1263,12 @@ static const struct ns16550_config __initconst uart_config[] =
.dev_id = 0x9910,
.param = param_asix
},
+ /* Amazon EC2 bare metal UART */
+ {
+ .vendor_id = PCI_VENDOR_ID_AMAZON,
+ .dev_id = 0x8250,
+ .param = param_amazon
+ },
};
static int __init
diff --git a/xen/include/xen/pci_ids.h b/xen/include/xen/pci_ids.h
index fd424ef55d..a17c88dcf7 100644
--- a/xen/include/xen/pci_ids.h
+++ b/xen/include/xen/pci_ids.h
@@ -17,6 +17,8 @@
#define PCI_VENDOR_ID_WCHIC 0x1c00
+#define PCI_VENDOR_ID_AMAZON 0x1d0f
+
#define PCI_VENDOR_ID_INTEL 0x8086
#endif /* XEN_PCI_IDS_H */
--
2.55.0On 23.09.2026 17:39, Benjamin Leggett wrote:
> Amazon EC2 bare metal instances have no UART at the legacy I/O port
> 0x3f8. Their only serial port is a 16550-compatible PCI device (vendor
> 0x1d0f, device 0x8250) with its registers in the MMIO space of BAR 0.
>
> pci_uart_config() uses the default parameters for a device that is not
> in uart_config[], and those only accept an I/O BAR, so "com1=...,pci"
> cannot find this device. Add it, with the same layout that Linux uses
> (commit 3bfd1300abfe ("serial: 8250_pci: Add Amazon PCI serial device
> ID")): one port at the start of BAR 0, 1-byte registers, and the
> default 1.8432MHz clock.
>
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Benjamin Leggett <benjamin@edera.io>
Acked-by: Jan Beulich <jbeulich@suse.com>
© 2016 - 2026 Red Hat, Inc.