Re: [kvm-unit-tests PATCH v2 12/18] scripts/runtime: Add default arguments for kvmtool

[Alexandru Elisei <alexandru.elisei@arm.com>]

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:
> > 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
> 
> > 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