Re: [PATCH v1] RISC-V: take text_mutex during alternative patching

["Heiko Stübner" <heiko@sntech.de>]

Am Mittwoch, 22. Februar 2023, 17:47:10 CET schrieb Palmer Dabbelt:
> On Wed, 22 Feb 2023 07:39:11 PST (-0800), heiko@sntech.de wrote:
> > Am Mittwoch, 22. Februar 2023, 16:31:25 CET schrieb Andrew Jones:
> >> On Wed, Feb 22, 2023 at 03:27:43PM +0100, Andrew Jones wrote:
> >> > On Thu, Feb 16, 2023 at 07:33:55AM -0800, Guenter Roeck wrote:
> >> > > On 2/15/23 14:00, Heiko Stübner wrote:
> >> > > [ ... ]
> >> > > > 
> >> > > > So now I've also tested Palmer's for-next at
> >> > > >    commit ec6311919ea6 ("Merge patch series "riscv: Optimize function trace"")
> >> > > > 
> >> > > > again with the same variants
> >> > > > - qemu-riscv32 without zbb
> >> > > > - qemu-riscv32 with zbb
> >> > > > - qemu-riscv64 without zbb
> >> > > > - qemu-riscv64 with zbb
> >> > > > 
> >> > > > And all of them booted fine into a nfs-root (debian for riscv64 and a
> >> > > > buildroot for riscv32).
> >> > > > 
> >> > > > I even forced a bug into the zbb code to make sure the patching worked
> >> > > > correctly (where the kernel failed as expected).
> >> > > > 
> >> > > > Qemu-version for me was 7.2.50 (v7.2.0-744-g5a3633929a-dirty)
> >> > > > 
> >> > > > I did try the one from Debian-stable (qemu-5.2) but that was too old and
> >> > > > didn't support Zbb yet.
> >> > > > 
> >> > > > One thing of note, the "active" 32bit config I had, somehow didn't produce
> >> > > > working images and I needed to start a new build using the rv32_defconfig.
> >> > > > 
> >> > > > So right now, I'm not sure what more to test though.
> >> > > > 
> >> > > 
> >> > > Another example:
> >> > > 
> >> > > - build defconfig
> >> > > - run
> >> > >   qemu-system-riscv64 -M virt -m 512M -no-reboot -kernel arch/riscv/boot/Image \
> >> > >     -snapshot -device virtio-blk-device,drive=d0 -drive file=rootfs.ext2,if=none,id=d0,format=raw \
> >> > >     -append "root=/dev/vda console=ttyS0,115200 earlycon=uart8250,mmio,0x10000000,115200" \
> >> > >     -nographic -monitor none
> >> > > 
> >> > > With CONFIG_RISCV_ISA_ZBB=y, that results in
> >> > > 
> >> > > [    0.818263] /dev/root: Can't open blockdev
> >> > > [    0.818856] VFS: Cannot open root device "/dev/vda" or unknown-block(0,0): error -6
> >> > > [    0.819177] Please append a correct "root=" boot option; here are the available partitions:
> >> > > [    0.819808] fe00           16384 vda
> >> > > [    0.819944]  driver: virtio_blk
> >> > > [    0.820534] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
> >> > > [    0.821101] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.2.0-rc8-next-20230216-00002-g80332825e240 #4
> >> > > [    0.821672] Hardware name: riscv-virtio,qemu (DT)
> >> > > [    0.822050] Call Trace:
> >> > > [    0.822427] [<ffffffff800053e4>] dump_backtrace+0x1c/0x24
> >> > > [    0.822834] [<ffffffff807f90e4>] show_stack+0x2c/0x38
> >> > > [    0.823085] [<ffffffff80803aea>] dump_stack_lvl+0x3c/0x54
> >> > > [    0.823351] [<ffffffff80803b16>] dump_stack+0x14/0x1c
> >> > > [    0.823601] [<ffffffff807f944c>] panic+0x102/0x29e
> >> > > [    0.823834] [<ffffffff80a015e2>] mount_block_root+0x18c/0x23e
> >> > > [    0.824148] [<ffffffff80a0187c>] mount_root+0x1e8/0x218
> >> > > [    0.824398] [<ffffffff80a019ee>] prepare_namespace+0x142/0x184
> >> > > [    0.824655] [<ffffffff80a01182>] kernel_init_freeable+0x236/0x25a
> >> > > [    0.824934] [<ffffffff80804602>] kernel_init+0x1e/0x10a
> >> > > [    0.825201] [<ffffffff80003560>] ret_from_exception+0x0/0x16
> >> > > [    0.826180] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---
> >> > > 
> >> > > This works fine if CONFIG_RISCV_ISA_ZBB is not enabled.
> >> > > 
> >> > > Tested with gcc 11.3, binutils 2.39, qemu v7.2.0 and qemu built from mainline.
> >> > >
> >> > 
> >> > Just to +1 this, I get the same result (unable to mount root fs) with
> >> > 
> >> > $QEMU -cpu rv64,zbb=on \
> >> >         -nographic \
> >> >         -machine virt \
> >> >         -kernel $KERNEL \
> >> >         -append 'root=/dev/vda console=ttyS0' \
> >> >         -drive file=disk.ext4,format=raw,id=hd0 \
> >> >         -device virtio-blk-pci,drive=hd0
> >> > 
> >> > kernel:   latest riscv-linux/for-next (8658db0a4a0f), defconfig
> >> > gcc:      riscv-gnu-toolchain (12.1.0)
> >> > binutils: riscv-gnu-toolchain (2.39)
> >> > qemu:     latest master (79b677d658d3)
> >> > 
> >> > Flipping the QEMU cpu zbb property off allows boot to succeed, i.e. it's
> >> > not necessary to compile out the CONFIG_RISCV_ISA_ZBB code from the
> >> > kernel, it's just necessary to avoid using it.
> >> >
> >> 
> >> Looks like something in the strncmp implementation. Only commenting it
> >> out allows boot to succeed.
> >
> > and interestingly it seems to be something very specific. I.e. my setup is
> > nfsroot-based (qemu is "just" another board in my boardfarm) and booting
> > into an nfs-root works quite nicely.
> >
> > I guess I need to look into how to get an actual disk-image in there.
> 
> It looks like Drew isn't using an initrd (and with NFS, presumably you 
> are)?  That's probably a big difference as well.

There is no initrd involved in my qemu setup ;-) .
Just a kernel built from current for-next + defconfig and a nfs-server
holding a full Debian-riscv64 system.


The magic qemu incantation is also pretty standard:

/usr/local/bin/qemu-system-riscv64 -M virt -smp 2 -m 1G -display none \
 -cpu rv64,zbb=true,zbc=true,svpbmt=true,Zicbom=true,Zawrs=true,sscofpmf=true,v=true \
 -serial telnet:localhost:5500,server,nowait -kernel /home/devel/nfs/kernel/riscv64/Image \
 -append earlycon=sbi root=/dev/nfs nfsroot=10.0.2.2:/home/devel/nfs/rootfs-riscv64virt ip=dhcp rw \
 -netdev user,id=n1 -device virtio-net-pci,netdev=n1 -name boardfarm-0,process=boardfarm-vm-0 -daemonize


And I also checked that my kernel really lands in the Zbb-code by simply
adding a "ret" [0] and making sure it breaks vs. runs without it


Next I'll try to move my rootfs into a real image-file and switch over to
the commandline Drew calls, to see if it'll reproduce then.



[0] 
diff --git a/arch/riscv/lib/strncmp.S b/arch/riscv/lib/strncmp.S
index ee49595075be..6b35933c5949 100644
--- a/arch/riscv/lib/strncmp.S
+++ b/arch/riscv/lib/strncmp.S
@@ -53,6 +53,7 @@ strncmp_zbb:
 .option push
 .option arch,+zbb
 
+       ret
        /*
         * Returns
         *   a0 - comparison result, like strncmp




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