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[0/3] hw/arm: Create second NonSecure UART for virt board

Message ID 20231023161532.2729084-1-peter.maydell@linaro.org
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Series hw/arm: Create second NonSecure UART for virt board | expand

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Peter Maydell Oct. 23, 2023, 4:15 p.m. UTC 
For some use-cases, it is helpful to have more than one UART available
to the guest, but the Arm 'virt' board only creates one.  If the
second UART slot is not already used for a TrustZone Secure-World-only
UART, create it as a NonSecure UART if the user provides a serial
backend for it (e.g. via a second -serial command line option).

We've wanted this for literally years; my first attempt at it
was this series in 2017:
https://lore.kernel.org/all/1512745328-5109-1-git-send-email-peter.maydell@linaro.org/
More recently  Axel Heider revived the idea with a patchset last year:
https://lore.kernel.org/qemu-devel/166990501232.22022.16582561244534011083-0@git.sr.ht/

However it has previously foundered on the problem that EDK2 does
odd things when presented with multiple UARTs in the DTB. (Specifically,
it will send the guest GRUB bootloader output to UART1, debug output
to both UARTs 0 and 1 depending on how far through boot it is, and the
guest kernel will use UART0 since that's what the ACPI tables say.)

Several things here I think mean we can finally get over this issue:
 * I learnt about the device tree "aliases" node; this allows us to
   explicitly say "this node is the first UART and this node is the
   second UART". So guests like Linux that follow this part of the
   DTB spec will always get the UART order correct; and if there are
   obscure guests that turn out to misbehave, we can point at the
   spec and say "this is how you should fix this on your end"...
 * This patch, like Axel's patch, only creates the second UART if
   the user asks for it on the command line, so any pre-existing
   command lines will not change behaviour.
 * Laszlo Ersek has kindly written some EDK2 patches that rationalise
   what it does when it finds more than one UART. This means that
   we can tell any users who do want to use two UARTs with EDK2
   "you should upgrade your EDK2 blobs to version NNN if you want to
   do that".

I haven't tagged this patchset RFC because I think it is good for
review as it stands. However I think we should ideally wait for the
EDK2 changes to go upstream first, so that we can fill in the TODO
comment in patch 3 and mention in our release notes what the new
EDK2 that correctly handles multiple UARTs would be. So I'm not
targeting this at 8.2.

Review from ACPI experts to confirm that I got the ACPI table
changes right in patch 3 would be appreciated :-)

thanks
-- PMM

Peter Maydell (3):
  hw/arm/virt: Add serial aliases in DTB
  hw/arm/virt: Rename VIRT_UART and VIRT_SECURE_UART to VIRT_UART[01]
  hw/arm/virt: allow creation of a second NonSecure UART

 docs/system/arm/virt.rst |  6 ++++-
 include/hw/arm/virt.h    |  5 ++--
 hw/arm/virt-acpi-build.c | 22 ++++++++++-------
 hw/arm/virt.c            | 52 +++++++++++++++++++++++++++++++++-------
 4 files changed, 65 insertions(+), 20 deletions(-)