Hi,

On Thu, Jan 23, 2025 at 03:07:18PM +0100, Andrew Jones wrote:

On Mon, Jan 20, 2025 at 04:43:10PM +0000, Alexandru Elisei wrote:

quoted

kvmtool, unless told otherwise, will do its best to make sure that a kernel
successfully boots in a virtual machine. Among things like automatically
creating a rootfs, it also adds extra parameters to the kernel command
line. This is actively harmful to kvm-unit-tests, because some tests parse
the kernel command line and they will fail if they encounter the options
added by kvmtool.

Fortunately for us, kvmtool commit 5613ae26b998 ("Add --nodefaults command
line argument") addded the --nodefaults kvmtool parameter which disables

added

quoted

all the implicit virtual machine configuration that cannot be disabled by
using other parameters, like modifying the kernel command line. Always use
--nodefaults to allow a test to run.

kvmtool can be too verbose when running a virtual machine, and this is
controlled with parameters. Add those to the default kvmtool command line
to reduce this verbosity to a minimum.

Before:

$ vm run arm/selftest.flat --cpus 2 --mem 256 --params "setup smp=2 mem=256"
  Info: # lkvm run -k arm/selftest.flat -m 256 -c 2 --name guest-5035
Unknown subtest

EXIT: STATUS=127
  Warning: KVM compatibility warning.
	virtio-9p device was not detected.
	While you have requested a virtio-9p device, the guest kernel did not initialize it.
	Please make sure that the guest kernel was compiled with CONFIG_NET_9P_VIRTIO=y enabled in .config.
  Warning: KVM compatibility warning.
	virtio-net device was not detected.
	While you have requested a virtio-net device, the guest kernel did not initialize it.
	Please make sure that the guest kernel was compiled with CONFIG_VIRTIO_NET=y enabled in .config.
  Info: KVM session ended normally.

After:

$ vm run arm/selftest.flat --nodefaults --network mode=none --loglevel=warning --cpus 2 --mem 256 --params "setup smp=2 mem=256"

On riscv I've noticed that with --nodefaults if I don't add parameters
with --params then kvmtool segfaults. I have to add --params "" to
avoid it. Does that also happen on arm? Anyway, that's something we
should fix in kvmtool rather than workaround it here.

This should fix it:

diff --git a/riscv/fdt.c b/riscv/fdt.c
index 85c8f95604f6..f6a702533258 100644
--- a/riscv/fdt.c
+++ b/riscv/fdt.c

@@ -256,9 +256,10 @@ static int setup_fdt(struct kvm *kvm)
                if (kvm->cfg.kernel_cmdline)
                        _FDT(fdt_property_string(fdt, "bootargs",
                                                 kvm->cfg.kernel_cmdline));
-       } else
+       } else if (kvm->cfg.real_cmdline) {
                _FDT(fdt_property_string(fdt, "bootargs",
                                         kvm->cfg.real_cmdline));
+       }

        _FDT(fdt_property_string(fdt, "stdout-path", "serial0"));

Looking at the timestamp on the commit, the patch that added --nodefaults
came before the patch that added riscv to kvmtool (by about a month). Just
in case you want to add a Fixes tag.

Thanks,
Alex