Re: [PATCH v8 00/26] x86: introduce NS16550-compatible UART emulator

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Hello Denis,

On 11/11/26 8:47 PM, dmukhin@xxxxxxxx wrote:
> x86 port of Xen lacks vUART facility similar to Arm's vpl011 to support x86
> guest OS bring up in the embedded setups.
>
> This patch series introduces initial in-hypervisor emulator for
> NS8250/NS16x50-compatible UARTs under CONFIG_VUART_NS16X50.
>
> In parallel domain creation scenario (hyperlaunch), NS16550 emulator helps
> early guest firmware and OS bringup debugging, because it eliminates
> dependency on the external emulator (qemu) being operational by the time
> domains are created.
>
> The emulator also allows to forward the physical console input to the x86
> domain which is useful when a system has only one physical UART for early
> debugging and this UART is owned by Xen.
>
> By default, CONFIG_VUART_NS16X50 enables emulation of NS16550 at I/O port
> 0x3f8, IRQ#4 in guest OS (legacy COM1). Legacy COM resources can be selected
> at run-time via temporary and undocumented switch 'vuart='.
>
> CONFIG_VUART_NS16X50_DEBUG enables some extra debugging facilities useful
> for NS16550 emulator development/debugging (disabled by default).
>
> The NS16550 emulator is disabled in default x86 configuration and goes under
> CONFIG_EXPERT in Kconfig.
>
> Note, there's a follow on work for RISC-V which is based on that series:
>    
> https://gitlab.com/xen-project/people/olkur/xen/-/tree/riscv-vuart-support-wip?ref_type=heads
>
> Limitations
> ===========
> - Only x86;
> - Only legacy COM resources, custom I/O ports/IRQs are not supported;
> - Only Xen console as a backend, no inter-domain communication (similar to
>    vpl011 on Arm);
> - Only 8n1 emulation (8-bit data, no parity, 1 stop bit);
> - No toolstack integration;
> - No baud rate emulation (reports 115200 baud to the guest OS);
> - No FIFO-less mode emulation;
> - No RX FIFO interrupt moderation (FCR) emulation;
> - No integration w/ VM snapshotting (HVM_REGISTER_SAVE_RESTORE() and
>    friends);
> - No MMIO-based UART emulation.
>
> Series
> ======
>
>    Patch 1 introduces the new vUART framework, that is the code originally
>    posted here:
>      
> https://lore.kernel.org/xen-devel/20250624035443.344099-16-dmukhin@xxxxxxxx/
>    Required for emulator.
>
>    Patch 2 adds missing NS16550 definitions, required for emulator.
>
>    Patches 3-4 introduces the basic emulator skeleton - state machine
>    initialization stubs, I/O port handler stub, logging, etc.
>
>    Patches 5-12 incrementally populate the minimal NS16550 register emulation.
>
>    Patch 13 hooks vUART state debugging (disabled by default).
>
>    Patch 14-16 adds Kconfig for enabling minimal NS16550 UART (disabled by 
> default).
>
>    Patches 17-21 introduce necessary changes to enable NS16550 on dom0 (and 
> PVH).
>
>    Patches 22-23 documentation updates. Note, that vuart= setting is not
>    documented since it will likely change with hyperlanch landing on mainline.
>
>    Patches 24-26 tests updates.
>
> Link to CI: 
> https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/3055921295
> Link to XTF branch: 
> https://gitlab.com/xen-project/people/dmukhin/xtf/-/commits/emul-16x50
> Link to Xen branch: 
> https://gitlab.com/xen-project/people/dmukhin/xen/-/tree/vuart-ns8250-v8?ref_type=heads
I built every patch of the series on x86 (defconfig + EXPERT + DEBUG,
gcc 16.2) and a few of them don't build:

- 18/26, 19/26, 20/26 with CONFIG_VUART_NS16X50=y: ns16x50_deinit() uses
  d, which is declared only in 21/26.

- 16/26 up to 26/26 with CONFIG_VUART_NS16X50_DEBUG=y: in
  ns16x50_dump_state() (13/26) the initializer of as dereferences info
  before info = vdev->info, so -Werror=uninitialized rejects it. It
  shows up only from 16/26 on because nothing compiles ns16x50.c before 
  the Kconfig option is introduced.
~ Oleksii