Re: [PATCH] target/riscv: don't enable Zfa by default

[Jerry ZJ]

We do have some cases that failed. SiFive e-series cores (https://static.dev.sifive.com/SiFive-E21-Manual-v1p0.pdf) do not have F extension (For example: rv32imc_zicsr_zifencei_zba_zbb). When we use the corresponding extension options to configure QEMU, i.e., rv32, i=true, m=true, a=true, c=true, Zicsr=true, Zifencei=true, zba=true, zbb=true, the QEMU will have the following error.
Zfa extension requires F extension

IMHO, we should not enable Zfa extension by default, especially when Zfa requires F to be enabled implicitly.

Best Regards,

Jerry ZJ

SiFive Inc. Taiwan

On Nov 6, 2023 at 22:55 +0800, Daniel Henrique Barboza <dbarboza@ventanamicro.com>, wrote:

On 11/6/23 08:14, Jerry Zhang Jian wrote:

- Zfa requires F, we should not assume all CPUs have F extension
support.

We do not have a case where this happen, do we? The default CPUs have F
enabled (see misa_ext_cfgs[] in target/riscv/tcg/tcg-cpu.c), so zfa being
enable isn't a problem for them. Vendor CPUs might not have F enabled, but
they don't use the default values for extensions, so they're not affected.
Having zfa enabled by default does not hurt the default CPU setups we have.

I am not a fan of these defaults for rv64 and so on, but once we set them to
'true' people can complain if we set them to 'false' because it might break
existing configs in the wild. We need a strong case (i.e. a bug) to do so.


Thanks,

Daniel

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>
---
target/riscv/cpu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index ac4a6c7eec..c9f11509c8 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -1247,7 +1247,7 @@ const RISCVCPUMultiExtConfig riscv_cpu_extensions[] = {
MULTI_EXT_CFG_BOOL("zihintntl", ext_zihintntl, true),
MULTI_EXT_CFG_BOOL("zihintpause", ext_zihintpause, true),
MULTI_EXT_CFG_BOOL("zawrs", ext_zawrs, true),
- MULTI_EXT_CFG_BOOL("zfa", ext_zfa, true),
+ MULTI_EXT_CFG_BOOL("zfa", ext_zfa, false),
MULTI_EXT_CFG_BOOL("zfh", ext_zfh, false),
MULTI_EXT_CFG_BOOL("zfhmin", ext_zfhmin, false),
MULTI_EXT_CFG_BOOL("zve32f", ext_zve32f, false),