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Re: [PATCH 2/2] target/riscv: Add short-isa-string option
From: |
Tsukasa OI |
Subject: |
Re: [PATCH 2/2] target/riscv: Add short-isa-string option |
Date: |
Tue, 10 May 2022 20:20:10 +0900 |
On 2022/05/09 18:51, Alistair Francis wrote:
> On Sun, Apr 24, 2022 at 7:22 AM Tsukasa OI <research_trasio@irq.a4lg.com>
> wrote:
>>
>> Because some operating systems don't correctly parse long ISA extension
>> string, this commit adds short-isa-string boolean option to disable
>> generating long ISA extension strings on Device Tree.
>>
>> Operating Systems which short-isa-string might be helpful:
>>
>> 1. Linux (5.17 or earlier)
>> 2. FreeBSD (at least 14.0-CURRENT)
>> 3. OpenBSD (at least current development version)
>>
>> Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
>> ---
>> target/riscv/cpu.c | 5 ++++-
>> target/riscv/cpu.h | 2 ++
>> 2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
>> index c765f7ff00..9718cd0e7e 100644
>> --- a/target/riscv/cpu.c
>> +++ b/target/riscv/cpu.c
>> @@ -834,6 +834,8 @@ static Property riscv_cpu_properties[] = {
>> DEFINE_PROP_BOOL("x-aia", RISCVCPU, cfg.aia, false),
>>
>> DEFINE_PROP_UINT64("resetvec", RISCVCPU, cfg.resetvec, DEFAULT_RSTVEC),
>> +
>> + DEFINE_PROP_BOOL("short-isa-string", RISCVCPU, cfg.short_isa_string,
>> false),
>> DEFINE_PROP_END_OF_LIST(),
>> };
>>
>> @@ -989,7 +991,8 @@ char *riscv_isa_string(RISCVCPU *cpu)
>> }
>> }
>> *p = '\0';
>> - riscv_isa_string_ext(cpu, &isa_str, maxlen);
>> + if (!cpu->cfg.short_isa_string)
>> + riscv_isa_string_ext(cpu, &isa_str, maxlen);
>
> I don't love this, the long strings are part of the ISA, it seems
> strange to add an option to disable them.
>
> Can you provide more details on what this breaks?
>
> Alistair
I don't like it either but I think this is necessary for (at least) a few
years (as a workaround).
Images for testing:
<https://a4lg.com/downloads/archives/tmp/2022-05-10/qemu-issue-reproduction-20220510.tar.xz>
Use latest (development version of) QEMU to reproduce.
- Linux 5.15 (FPU support enabled)
- Busybox 1.35.0 (use of FPU disabled, -march=rv64imac -mabi=lp64)
Config 1. `-cpu rv64,g=on,f=on,d=on,zfinx=off,zdinx=off'
This is generic RV64.
ISA string is "rv64imafdch_zba_zbb_zbc_zbs".
With this ISA, it works. ...Actually, it misunderstands Zbc extension as
`Z', `B' and `C' extensions (which might cause problems on other
configurations) in Linux 5.15 but... not now.
Config 2. `-cpu rv64,g=off,f=off,d=off,zfinx=on,zdinx=on'
This is generic RV64 but with floating point using GPRs (Zfinx and Zdinx).
ISA string is "rv64imach_zfinx_zdinx_zba_zbb_zbc_zbs".
OK, this is the problem. If you try to run userland (Busybox-based), it
crashes on __fstate_restore function in kernel.
[ 0.619174] Oops - illegal instruction [#1]
[ 0.619544] Modules linked in:
[ 0.619913] CPU: 0 PID: 1 Comm: init Not tainted 5.15.0 #47
[ 0.620594] Hardware name: riscv-virtio,qemu (DT)
[ 0.621142] epc : __fstate_restore+0x12/0x8c
[ 0.621858] ra : start_thread+0x28/0x5a
[ 0.623463] epc : ffffffff80005332 ra : ffffffff80003352 sp :
ffffffd00060bc90
[ 0.624291] gp : ffffffff812e6e38 tp : ffffffe001630000 t0 :
0000000000000000
[ 0.625194] t1 : 0000000000006000 t2 : 0000000000000000 s0 :
ffffffd00060bcc0
[ 0.626448] s1 : ffffffd00060bee0 a0 : ffffffe001630900 a1 :
000000000001054c
[ 0.627431] a2 : 0000000000000900 a3 : 0000000000000000 a4 :
0000000000000000
[ 0.627983] a5 : 0000000000002020 a6 : 000000000000000c a7 :
0000000000000000
[ 0.629473] s2 : 0000003ff4473e10 s3 : 000000000001054c s4 :
0000003ff4473ff2
[ 0.630798] s5 : 0000003ffffffff8 s6 : 000000000001054c s7 :
0000000000040000
[ 0.631623] s8 : 0000003ff4473e38 s9 : 0000003ff4473e38 s10:
ffffffe002083600
[ 0.632310] s11: ffffffe002070000 t3 : 000000000000000e t4 :
0000000000000000
[ 0.633080] t5 : 0000000000000180 t6 : 0000000000040000
[ 0.633648] status: 0000000200000120 badaddr: 0000000000053007 cause:
0000000000000002
[ 0.635025] [<ffffffff80005332>] __fstate_restore+0x12/0x8c
[ 0.635771] [<ffffffff8017eb1a>] load_elf_binary+0xe16/0xe4a
[ 0.636149] [<ffffffff8012c97a>] bprm_execve+0x1e4/0x468
[ 0.636603] [<ffffffff8012d646>] kernel_execve+0xdc/0x142
[ 0.636943] [<ffffffff80709158>] run_init_process+0x90/0x9e
[ 0.637493] [<ffffffff807136a2>] kernel_init+0x72/0x104
[ 0.638390] [<ffffffff80003008>] ret_from_exception+0x0/0xc
[ 0.639513] ---[ end trace e4dec1a155401c43 ]---
[ 0.640489] Kernel panic - not syncing: Attempted to kill init!
exitcode=0x0000000b
Apparently, it crashes as follows:
1. Linux (-5.17) misunderstands `Zfinx' and `Zdinx' extensions as I, F, D,
N, X and Z single-letter extensions and thinks FPU with dedicated
registers is there.
2. Because of that, the kernel tries to initialize FP registers from
memory using `fld' instruction but this is a part of `D' extension,
not `Zdinx'.
3. Illegal instruction trap is generated and the kernel panics.
As you can see, many operating systems currently in use still don't
correctly understand long ISA strings:
>> 1. Linux (5.17 or earlier)
>> 2. FreeBSD (at least 14.0-CURRENT)
>> 3. OpenBSD (at least current development version)
...and it affects in-kernel behavior directly! That means, we still need
something to prevent multi-letter extension names from appearing in
"riscv,isa" DeviceTree ISA string. That's the purpose of this option.
I am preparing for PATCH v2 (which "moves" Zhinx*, instead of removing) so
please wait for it (this commit will be unchanged but will reflect your
comment).
Tsukasa
>
>> return isa_str;
>> }
>>
>> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
>> index 34c22d5d3b..5b7fe32218 100644
>> --- a/target/riscv/cpu.h
>> +++ b/target/riscv/cpu.h
>> @@ -408,6 +408,8 @@ struct RISCVCPUConfig {
>> bool aia;
>> bool debug;
>> uint64_t resetvec;
>> +
>> + bool short_isa_string;
>> };
>>
>> typedef struct RISCVCPUConfig RISCVCPUConfig;
>> --
>> 2.32.0
>>
>