Re: [PATCH v6 00/15] x86: introduce NS16550-compatible UART emulator

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Oleksii and all,

I would like to consider patches 1-12 of this patch series for 4.21,
pending the few minor comments I made addressed.


On Fri, 5 Sep 2025, dmukhin@xxxxxxx 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
> 0x2f8, IRQ#3 in guest OS (legacy COM2). Legacy COM resources cannot be
> selected at built-time or via per-domain xl configuration in this initial
> submission.
> 
> 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.
> 
> Limitations
> ===========
> - Only x86;
> - Only legacy COM2 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.
> 
>   Patch 3 introduces the basic emulator skeleton - state machine
>   initialization stubs, I/O port handler stub, logging, etc.
> 
>   Patches 4-11 incrementally populate the minimal NS16550 register emulation.
> 
>   Patch 12 hooks vUART state debugging (disabled by default).
> 
>   Pathes 13-15 introduce necessary changes to enable NS16550 on dom0 (and 
> PVH).
> 
> Link to CI: 
> https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2024756493
> Link to branch: 
> https://gitlab.com/xen-project/people/dmukhin/xen/-/tree/vuart-ns8250-v6?ref_type=heads
> 
> Testing
> =======
> 
>   ```shell
>   echo CONFIG_EXPERT=y >> .config
>   echo CONFIG_VUART_NS16X50=y >> .config
>   make olddefconfig
>   ```
>   COM2 (0x2f8) resources are used by default.
> 
>   To test w/ virtual COM2, the guest kernel parameters should contain
>   something like the following:
>     earlycon=uart,io,0x2f8,115200n8 console=uart,io,0x2f8,115200n8
> 
>   HVM
>   ---
>   Tested only boot of HVM linux guest with OVMF as the virtual firmware.
>   SeaBIOS as a virtual firmware is not tested.
> 
>   PVH (dom0)
>   ----------
>   Xen is able to forward physical console input to the domain with virtual
>   NS16550. To switch the console focus press Ctrl+aaa.
>   Console switch is limited on x86 to dom0 and Xen (fixes pending).
> 
> Changes since v5:
> - Split THR/RBR into two separate patches.
> - Addressed feedback from v5.
> - Link to v5: 
> https://lore.kernel.org/xen-devel/20250828235409.2835815-1-dmukhin@xxxxxxxx/
> 
> Changes since v4:
> - Split the series to make it simpler to review.
> - Addressed feedback from v4.
> - Dropped xl changes, which I will submit separately.
> - Link to v4: 
> https://lore.kernel.org/xen-devel/20250731192130.3948419-1-dmukhin@xxxxxxxx/
> 
> Changes since v3:
> - Reduced the blast radius of the series, thanks to reviews, individual
>   aspects (like console focus) touched in v3 moved to separate threads.
> - Kept the UART emulator framework since I need to redo some of emulator code
>   and there's more-or-less agreement on it (where to place, naming, scope).
> - Applied the feedback from
>     
> https://lore.kernel.org/xen-devel/20250624035443.344099-1-dmukhin@xxxxxxxx/
> - Link to v3: 
> https://lore.kernel.org/xen-devel/20250103-vuart-ns8250-v3-v1-0-c5d36b31d66c@xxxxxxxx/
> 
> Changes since v2:
> - renamed emulator s/NS8250/NS16550/g
> - reduced the patch series after addressing v2 feedback
> - introduced driver framework for UART emulators
> - unified guest OS printouts across all available UART emulators
> - Link to v2: 
> https://lore.kernel.org/xen-devel/20241205-vuart-ns8250-v1-0-e9aa923127eb@xxxxxxxx/
> 
> Changes since v1:
> - dropped kmalloc/kfree aliases
> - fixed ECLAIR jobs (thanks Andrew Cooper)
> - addressed console forwarding on arm32 and arm64 (thanks to Luca Fancellu)
> - moved NS8250 debugging stubs into its own patch
> - added fix for https://gitlab.com/xen-project/xen/-/issues/184
> - Link to v1: 
> https://lore.kernel.org/r/20241126-vuart-ns8250-v1-v1-0-87b9a8375b7a@xxxxxxxx
> 
> Denis Mukhin (15):
>   emul/vuart: introduce framework for UART emulators
>   xen/8250-uart: update definitions
>   emul/ns16x50: implement emulator stub
>   emul/ns16x50: implement DLL/DLM registers
>   emul/ns16x50: implement SCR register
>   emul/ns16x50: implement IER/IIR registers
>   emul/ns16x50: implement LCR/LSR registers
>   emul/ns16x50: implement MCR/MSR registers
>   emul/ns16x50: implement RBR register
>   emul/ns16x50: implement THR register
>   emul/ns16x50: implement FCR register (write-only)
>   emul/ns16550: implement dump_state() hook
>   x86/domain: enable per-domain I/O port bitmaps
>   xen/domain: allocate d->irq_caps before arch-specific initialization
>   emul/ns16x50: implement IRQ emulation via vIOAPIC
> 
>  xen/arch/arm/xen.lds.S                   |   1 +
>  xen/arch/ppc/xen.lds.S                   |   1 +
>  xen/arch/riscv/xen.lds.S                 |   1 +
>  xen/arch/x86/Makefile                    |   1 +
>  xen/arch/x86/dom0_build.c                | 112 +--
>  xen/arch/x86/hvm/dom0_build.c            |   7 +
>  xen/arch/x86/hvm/hvm.c                   |  56 +-
>  xen/arch/x86/hvm/nestedhvm.c             |   8 +-
>  xen/arch/x86/hvm/quirks.c                |   3 -
>  xen/arch/x86/hvm/svm/nestedsvm.c         |   2 +-
>  xen/arch/x86/hvm/vioapic.c               |  10 +
>  xen/arch/x86/hvm/vmx/vvmx.c              |   4 +-
>  xen/arch/x86/include/asm/hvm/nestedhvm.h |   3 +-
>  xen/arch/x86/include/asm/hvm/support.h   |   2 -
>  xen/arch/x86/include/asm/iocap.h         |   2 +
>  xen/arch/x86/include/asm/irq.h           |   8 +
>  xen/arch/x86/ioport.c                    | 163 ++++
>  xen/arch/x86/irq.c                       |   8 +
>  xen/arch/x86/pv/dom0_build.c             |   7 +
>  xen/arch/x86/xen.lds.S                   |   1 +
>  xen/common/Kconfig                       |   2 +
>  xen/common/Makefile                      |   1 +
>  xen/common/domain.c                      |   8 +-
>  xen/common/emul/Kconfig                  |   6 +
>  xen/common/emul/Makefile                 |   1 +
>  xen/common/emul/vuart/Kconfig            |  25 +
>  xen/common/emul/vuart/Makefile           |   2 +
>  xen/common/emul/vuart/ns16x50.c          | 984 +++++++++++++++++++++++
>  xen/common/emul/vuart/vuart.c            | 157 ++++
>  xen/common/keyhandler.c                  |   3 +
>  xen/drivers/char/console.c               |   6 +-
>  xen/drivers/char/ns16550.c               |  16 +-
>  xen/drivers/passthrough/x86/hvm.c        |  11 +-
>  xen/include/xen/8250-uart.h              |  50 +-
>  xen/include/xen/sched.h                  |   4 +
>  xen/include/xen/serial.h                 |   3 +
>  xen/include/xen/vuart.h                  | 116 +++
>  xen/include/xen/xen.lds.h                |  10 +
>  38 files changed, 1634 insertions(+), 171 deletions(-)
>  create mode 100644 xen/arch/x86/ioport.c
>  create mode 100644 xen/common/emul/Kconfig
>  create mode 100644 xen/common/emul/Makefile
>  create mode 100644 xen/common/emul/vuart/Kconfig
>  create mode 100644 xen/common/emul/vuart/Makefile
>  create mode 100644 xen/common/emul/vuart/ns16x50.c
>  create mode 100644 xen/common/emul/vuart/vuart.c
>  create mode 100644 xen/include/xen/vuart.h
> 
> -- 
> 2.51.0
>