Re: [PATCH v1] s390x/tcg: Fix RISBHG

[David Hildenbrand]

On 08.01.21 11:19, David Hildenbrand wrote:
> On 08.01.21 10:44, David Hildenbrand wrote:
>> On 08.01.21 03:20, Nick Desaulniers wrote:
>>> On Thu, Jan 7, 2021 at 3:27 PM David Hildenbrand <dhildenb@redhat.com> 
>>> wrote:
>>>>
>>>>
>>>>> Am 08.01.2021 um 00:21 schrieb Nick Desaulniers <ndesaulniers@google.com>:
>>>>>
>>>>> On Thu, Jan 7, 2021 at 3:13 PM David Hildenbrand <david@redhat.com> 
>>>>> wrote:
>>>>>>
>>>>>> RISBHG is broken and currently hinders clang builds of upstream kernels
>>>>>> from booting: the kernel crashes early, while decompressing the image.
>>>>>>
>>>>>>  [...]
>>>>>>   Kernel fault: interruption code 0005 ilc:2
>>>>>>   Kernel random base: 0000000000000000
>>>>>>   PSW : 0000200180000000 0000000000017a1e
>>>>>>         R:0 T:0 IO:0 EX:0 Key:0 M:0 W:0 P:0 AS:0 CC:2 PM:0 RI:0 EA:3
>>>>>>   GPRS: 0000000000000001 0000000c00000000 00000003fffffff4 
>>>>>> 00000000fffffff0
>>>>>>         0000000000000000 00000000fffffff4 000000000000000c 
>>>>>> 00000000fffffff0
>>>>>>         00000000fffffffc 0000000000000000 00000000fffffff8 
>>>>>> 00000000008e25a8
>>>>>>         0000000000000009 0000000000000002 0000000000000008 
>>>>>> 000000000000bce0
>>>>>>
>>>>>> One example of a buggy instruction is:
>>>>>>
>>>>>>    17dde:       ec 1e 00 9f 20 5d       risbhg  %r1,%r14,0,159,32
>>>>>>
>>>>>> With %r14 = 0x9 and %r1 = 0x7 should result in %r1 = 0x900000007, 
>>>>>> however,
>>>>>> results in %r1 = 0.
>>>>>>
>>>>>> Let's interpret values of i3/i4 as documented in the PoP and make
>>>>>> computation of "mask" only based on i3 and i4 and use "pmask" only at the
>>>>>> very end to make sure wrapping is only applied to the high/low 
>>>>>> doubleword.
>>>>>>
>>>>>> With this patch, I can successfully boot a v5.10 kernel built with
>>>>>> clang, and gcc builds keep on working.
>>>>>>
>>>>>> Fixes: 2d6a869833d9 ("target-s390: Implement RISBG")
>>>>>> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
>>>>>> Cc: Guenter Roeck <linux@roeck-us.net>
>>>>>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>> ---
>>>>>>
>>>>>> This BUG was a nightmare to debug and the code a nightmare to understand.
>>>>>>
>>>>>> To make clang/gcc builds boot, the following fix is required as well on
>>>>>> top of current master: "[PATCH] target/s390x: Fix ALGSI"
>>>>>> 20210107202135.52379-1-david@redhat.com">https://lkml.kernel.org/r/20210107202135.52379-1-david@redhat.com
>>>>>
>>>>> In that case, a huge thank you!!! for this work! ++beers_owed.
>>>>>
>>>>
>>>> :) a kernel build for z13 should work with the (default) „-cpu qemu“ cpu 
>>>> type.
>>>
>>> Hmm...so I don't think clang can build a Linux kernel image with
>>> CONFIG_MARCH_Z13=y just yet; just defconfig.  Otherwise looks like
>>> clang barfs on some of the inline asm constraints.
>>>
>>
>> Ah, right. I overwrote my manual config by a temporary defconfig :)
>>
>>
>> So, I'm on x86-64 F33.
>>
>> clang version 11.0.0 (Fedora 11.0.0-2.fc33)
>> LLVM version 11.0.0
>>
>> I cannot directly use "LLVM=1" for cross-compilation, as I keep getting
>> "error: unknown emulation: elf64_s390" from ld.lld and "error: invalid
>> output format: 'elf64-s390'" from llvm-objcopy. I assume that's fixed in
>> llvm12?
>>
>> 1. I patch around it (strange, I remember CC= .. used to work, but it no
>> longer does)
>>
>> ---
>>
>> index e30cf02da8b8..89c57062ed5d 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -427,13 +427,13 @@ KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
>>  CPP            = $(CC) -E
>>  ifneq ($(LLVM),)
>>  CC             = clang
>> -LD             = ld.lld
>> -AR             = llvm-ar
>> -NM             = llvm-nm
>> -OBJCOPY                = llvm-objcopy
>> -OBJDUMP                = llvm-objdump
>> -READELF                = llvm-readelf
>> -STRIP          = llvm-strip
>> +LD             = $(CROSS_COMPILE)ld
>> +AR             = $(CROSS_COMPILE)ar
>> +NM             = $(CROSS_COMPILE)nm
>> +OBJCOPY                = $(CROSS_COMPILE)objcopy
>> +OBJDUMP                = $(CROSS_COMPILE)objdump
>> +READELF                = $(CROSS_COMPILE)readelf
>> +STRIP          = $(CROSS_COMPILE)strip
>>  else
>>  CC             = $(CROSS_COMPILE)gcc
>>  LD             = $(CROSS_COMPILE)ld
>>
>> ---
>>
>> 2. Compile using clang
>>
>>
>> Using latest linux-next (1c925d2030afd354a02c23500386e620e662622b) +
>> above patch
>>
>> ---
>>
>> #!/bin/bash
>> export ARCH=s390;
>> export CROSS_COMPILE=s390x-linux-gnu-
>> export LLVM=1
>> make distclean
>> make defconfig
>>
>> # Make F32 initrd boot without inserting modules
>> ./scripts/config -e CONFIG_SCSI_ISCSI_ATTRS
>> ./scripts/config -e CONFIG_ISCSI_TCP
>>
>> make -j40 > /dev/null
>>
>> ---
>>
>> 3. Run it via QEMU. I boot a full Fedora 32 using the cloud-image +
>> initrd from Fedora 32 (tried to stick to your cmdline where possible)
>>
>> ./build/qemu-system-s390x \
>> -m 512M \
>> -cpu qemu \
>> -display none \
>> -nodefaults \
>> -kernel ../linux-cross/arch/s390/boot/bzImage \
>> -append "root=/dev/vda1 conmode=sclp console=ttyS0" \
>> -initrd ../Fedora-Cloud-Base-32-1.6.x86_64-initrd.img \
>> -hda ../Fedora-Cloud-Base-32-1.6.x86_64-initrd.img \
> 
> ^ -hda ../Fedora-Cloud-Base-32-1.6.x86_64.qcow2 \
> 
> 

Using homebrew llvm-12/clang-12 results in a kernel that is indeed stuck
is in an endless program exception loop, seeming to try printing a
backtrace over and over again. Yet, no output besides

"KASLR disabled: CPU has no PRNG"

Kernel runs under KVM, so more debugging to do to get clang-12 builds
running as well.

-- 
Thanks,

David / dhildenb