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

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

Hi Denis and Stefano

I’d like to acknowledge the significant effort that went into this patch
series -- it’s clear that a lot of work has been invested.

On Sat, Sep 6, 2025 at 5:02 AM Stefano Stabellini
<sstabellini@xxxxxxxxxx> wrote:
>
> 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.

Although I am neither a maintainer nor an official reviewer for this
project, I have looked over some of the first patches in the series. In my
opinion, the series is not yet ready for merging.

Even if my review is set aside, the changes are largely x86-specific and
produce the most impact on this architecture. I believe that before
merging, one of the x86 maintainers (or at least a trusted reviewer for
x86, if available) should carefully review these patches.

>
>
> 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
> >
>

Best regards,
Mykola