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

[Denis Mukhin <dmkhn@xxxxxxxxx>]

On Tuesday, November 26th, 2024 at 11:26 PM, Jan Beulich <jbeulich@xxxxxxxx> 
wrote:

> 
> 
> On 27.11.2024 00:57, Andrew Cooper 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.
> 
> 
> +1
> 
> > Two things stand out at a glance.
> > 
> > First, xmalloc/free are the expected functions to use,
> 
> 
> Small correction - as of their introduction, it's xvmalloc() etc and xvfree()
> which are to be used in new code.

Addressed in v2.

> 
> One other, more general remark: Please send patches To: the list, with
> maintainers Cc-ed. And please compose the Cc: lists per-patch - looking at
> patch 2, there should really have been just a single person (Daniel) on Cc.
> 
> Jan