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 */
-- 
2.55.0

On 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>