[PATCH 3/3] hw/arm/virt: allow creation of a second NonSecure UART

[Peter Maydell]

For some use-cases, it is helpful to have more than one UART
available to the guest. If the second UART slot is not already
used for a TrustZone Secure-World-only UART, create it as a
NonSecure UART only when the user provides a serial backend
(e.g. via a second -serial command line option).

This avoids problems where existing guest software only expects
a single UART, and gets confused by the second UART in the DTB.
The major example of this is older EDK2 firmware, which will
send the GRUB bootloader output to UART1 and the guest
serial output to UART0. Users who want to use both UARTs
with a guest setup including EDK2 are advised to update
to a newer EDK2.

TODO: give specifics of which EDK2 version has this fix,
once the patches which fix EDK2 are upstream.

Inspired-by: Axel Heider <axel.heider@hensoldt.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
This patch was originally based on the one from Axel Heider
that aimed to do the same thing:
https://lore.kernel.org/qemu-devel/166990501232.22022.16582561244534011083-1@git.sr.ht/
but by the time I had added the ACPI support and dealt with
the EDK2 compatibility awkwardness, I found I had pretty
much rewritten it. So this combination of author and tags
seemed to me the most appropriate, but I'm happy to adjust
if people (esp. Axel!) would prefer otherwise.

It is in theory possible to slightly work around the
incorrect behaviour of old EDK2 binaries by listing the
two UARTs in the opposite order in the DTB. However since
old EDK2 ends up using the two UARTs in different orders
depending on which phase of boot it is in (and in particular
with EDK2 debug builds debug messages go to a mix of both
UARTs) this doesn't seem worthwhile. I think most users
who are interested in the second UART are likely to be
using a bare-metal or direct Linux boot anyway.
---
 docs/system/arm/virt.rst |  6 +++++-
 include/hw/arm/virt.h    |  1 +
 hw/arm/virt-acpi-build.c | 12 ++++++++----
 hw/arm/virt.c            | 38 +++++++++++++++++++++++++++++++++++---
 4 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
index e1697ac8f48..028d2416d5b 100644
--- a/docs/system/arm/virt.rst
+++ b/docs/system/arm/virt.rst
@@ -26,7 +26,7 @@ The virt board supports:
 
 - PCI/PCIe devices
 - Flash memory
-- One PL011 UART
+- Either one or two PL011 UARTs for the NonSecure World
 - An RTC
 - The fw_cfg device that allows a guest to obtain data from QEMU
 - A PL061 GPIO controller
@@ -48,6 +48,10 @@ The virt board supports:
   - A secure flash memory
   - 16MB of secure RAM
 
+The second NonSecure UART only exists if a backend is configured
+explicitly (e.g. with a second -serial command line option) and
+TrustZone emulation is not enabled.
+
 Supported guest CPU types:
 
 - ``cortex-a7`` (32-bit)
diff --git a/include/hw/arm/virt.h b/include/hw/arm/virt.h
index 0de58328b2f..da15eb342bd 100644
--- a/include/hw/arm/virt.h
+++ b/include/hw/arm/virt.h
@@ -150,6 +150,7 @@ struct VirtMachineState {
     bool ras;
     bool mte;
     bool dtb_randomness;
+    bool second_ns_uart_present;
     OnOffAuto acpi;
     VirtGICType gic_version;
     VirtIOMMUType iommu;
diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 54f26640982..b812f33c929 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -77,11 +77,11 @@ static void acpi_dsdt_add_cpus(Aml *scope, VirtMachineState 
*vms)
 }
 
 static void acpi_dsdt_add_uart(Aml *scope, const MemMapEntry *uart_memmap,
-                                           uint32_t uart_irq)
+                               uint32_t uart_irq, int uartidx)
 {
-    Aml *dev = aml_device("COM0");
+    Aml *dev = aml_device("COM%d", uartidx);
     aml_append(dev, aml_name_decl("_HID", aml_string("ARMH0011")));
-    aml_append(dev, aml_name_decl("_UID", aml_int(0)));
+    aml_append(dev, aml_name_decl("_UID", aml_int(uartidx)));
 
     Aml *crs = aml_resource_template();
     aml_append(crs, aml_memory32_fixed(uart_memmap->base,
@@ -860,7 +860,11 @@ build_dsdt(GArray *table_data, BIOSLinker *linker, 
VirtMachineState *vms)
     scope = aml_scope("\\_SB");
     acpi_dsdt_add_cpus(scope, vms);
     acpi_dsdt_add_uart(scope, &memmap[VIRT_UART0],
-                       (irqmap[VIRT_UART0] + ARM_SPI_BASE));
+                       (irqmap[VIRT_UART0] + ARM_SPI_BASE), 0);
+    if (vms->second_ns_uart_present) {
+        acpi_dsdt_add_uart(scope, &memmap[VIRT_UART1],
+                           (irqmap[VIRT_UART1] + ARM_SPI_BASE), 1);
+    }
     if (vmc->acpi_expose_flash) {
         acpi_dsdt_add_flash(scope, &memmap[VIRT_FLASH]);
     }
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index fd524aed6b6..7f60df7d7b2 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -856,7 +856,7 @@ static void create_gic(VirtMachineState *vms, MemoryRegion 
*mem)
 }
 
 static void create_uart(const VirtMachineState *vms, int uart,
-                        MemoryRegion *mem, Chardev *chr)
+                        MemoryRegion *mem, Chardev *chr, bool secure)
 {
     char *nodename;
     hwaddr base = vms->memmap[uart].base;
@@ -894,6 +894,8 @@ static void create_uart(const VirtMachineState *vms, int 
uart,
         qemu_fdt_setprop_string(ms->fdt, "/aliases", "serial0", nodename);
     } else {
         qemu_fdt_setprop_string(ms->fdt, "/aliases", "serial1", nodename);
+    }
+    if (secure) {
         /* Mark as not usable by the normal world */
         qemu_fdt_setprop_string(ms->fdt, nodename, "status", "disabled");
         qemu_fdt_setprop_string(ms->fdt, nodename, "secure-status", "okay");
@@ -2269,11 +2271,41 @@ static void machvirt_init(MachineState *machine)
 
     fdt_add_pmu_nodes(vms);
 
-    create_uart(vms, VIRT_UART0, sysmem, serial_hd(0));
+    /*
+     * The first UART always exists. If the security extensions are
+     * enabled, the second UART also always exists. Otherwise, it only exists
+     * if a backend is configured explicitly via '-serial <backend>'.
+     * This avoids potentially breaking existing user setups that expect
+     * only one NonSecure UART to be present (for instance, older EDK2
+     * binaries).
+     *
+     * The nodes end up in the DTB in reverse order of creation, so we must
+     * create UART0 last to ensure it appears as the first node in the DTB,
+     * for compatibility with guest software that just iterates through the
+     * DTB to find the first UART, as older versions of EDK2 do.
+     * DTB readers that follow the spec, as Linux does, should honour the
+     * aliases node information and /chosen/stdout-path regardless of
+     * the order that nodes appear in the DTB.
+     *
+     * For similar back-compatibility reasons, if UART1 is the secure UART
+     * we create it second (and so it appears first in the DTB), because
+     * that's what QEMU has always done.
+     */
+    if (!vms->secure) {
+        Chardev *serial1 = serial_hd(1);
+
+        if (serial1) {
+            vms->second_ns_uart_present = true;
+            create_uart(vms, VIRT_UART1, sysmem, serial1, false);
+        }
+    }
+    create_uart(vms, VIRT_UART0, sysmem, serial_hd(0), false);
+    if (vms->secure) {
+        create_uart(vms, VIRT_UART1, secure_sysmem, serial_hd(1), true);
+    }
 
     if (vms->secure) {
         create_secure_ram(vms, secure_sysmem, secure_tag_sysmem);
-        create_uart(vms, VIRT_UART1, secure_sysmem, serial_hd(1));
     }
 
     if (tag_sysmem) {
-- 
2.34.1