Re: [PATCH 00/36] Introduce NS8250 UART emulator

[Denis Mukhin <dmkhn@xxxxxxxxx>]

On Tuesday, November 26th, 2024 at 3:57 PM, Andrew Cooper 
<andrew.cooper3@xxxxxxxxxx> wrote:

> 
> 
> On 26/11/2024 11:21 pm, Denis Mukhin via B4 Relay wrote:
> 
> > The patch series introduces initial in-hypervisor emulator for
> > NS8250/NS16x50-compatible UARTs under CONFIG_HAS_VUART_NS8250.
> > 
> > In parallel domain creation scenario (hyperlaunch), NS8520 emulator helps
> > early guest OS bringup debugging, because it eliminates dependency on the
> > external emulator being operational by the time domains are created. Also,
> > there's no early console facility similar to vpl011 to support x86 guest OS
> > bring up.
> > 
> > The NS8250 emulator is disabled by default.
> > 
> > Series
> > ======
> > - patches 1-2: some miscellaneous random fixes, added into the series
> > because I stepped into those while debugging NS8250 emulator.
> > 
> > - patches 3-14: preparation fixes for xen console and NS8250 emulator.
> > 
> > - patches 15-29: xen console driver cleanup and preparation for enabling
> > physical serial console focus assignment to the guest VM w/ virtual NS8250.
> > 
> > - patches 30-36: initial NS8250 emulator. That adds the I/O port emulator
> > for legacy PC COM UARTs, Kconfig option, enabling emulator and libxl
> > plumbing.
> > 
> > Limitations
> > ===========
> > - Only x86;
> > - Only Linux guest tested so far;
> > - Only legacy COM{1,2,3,4} resources, no customization;
> > - Only Xen console as a backend, no inter-domain communication (similar to
> > vpl011 on Arm);
> > - Only 8-bit characters;
> > - Baud rate is not emulated;
> > - FIFO-less mode is not emulated properly;
> > - RX FIFO interrupt moderation (FCR) is not emulated properly, TL16C750
> > has special FCR handling;
> > - No integration w/ VM snapshotting (HVM_REGISTER_SAVE_RESTORE() and
> > friends);
> > - Assumes no ISA-device IRQ sharing;
> > - MMIO-based UART is not supported.
> > 
> > Testing
> > =======
> > 
> > I tested boot of HVM linux guest w/ OVMF as the virtual firmware.
> > 
> > The emulator, if enabled via CONFIG_HAS_VUART_NS8250=y, will use COM1 
> > (0x3f8)
> > resources by default.
> > 
> > To test w/ virtual COM1, the guest kernel parameters should contain
> > earlycon=uart,io,0x3f8,115200n8 console=uart,io,0x3f8,115200n8
> > 
> > Xen is able to forward physical console input to the domain w/ virtual 
> > NS8250.
> > To switch the console focus press Ctrl+aaa. If console= is given to the HVM
> > kernel, then the user shall be able to see the login prompt on xen console 
> > once
> > console focus is switched to the HVM guest.
> > 
> > Signed-off-by: Denis Mukhin dmukhin@xxxxxxxx
> 
> 
> Hello,
> 
> Thankyou for this. It's an excellent start for first posting to the list.
> 
> Two things stand out at a glance.
> 
> First, xmalloc/free are the expected functions to use, and kmalloc/free
> are not equivalent. We have some drivers ported from Linux, hence the
> compatibility, and if it needs to extend beyond smmu.c then there's
> linux-compat.h where definitions can live, but we really don't want them
> visible generally.

Dropped in v2:
  
https://lore.kernel.org/xen-devel/20241205-vuart-ns8250-v1-0-e9aa923127eb@xxxxxxxx/

> 
> Second and more importantly, I'm afraid this won't pass CI right now.
> Your function pointees (e.g. ns8250_iir_check_lsi()/etc) need a cf_check
> attribute on them, in order to function when running on CET-IBT capable
> hardware.
> 
> From the root of the Xen tree, if you run:
> 
> CONTAINER=bookworm-x86_64-gcc-ibt ./automation/scripts/containerize
> 
> then you'll get a build environment with suitable diagnostics.
> Unfortunately we're still relying on an out-of-tree GCC patch to have a
> compiler that can point out the problems at build time, as opposed to
> encountering the #CP exceptions at runtime.

Addressed in v2.
Thanks for help w/ fixing CI!

> 
> 
> I'll try to have a closer look at the rest of the series tomorrow.
> 
> ~Andrew