Re: [PATCH 02/10] gdbstub: introduce target independent gdb register helper

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 3/20/25 12:30, Pierrick Bouvier wrote:
> On 3/19/25 11:22, Alex Bennée wrote:
>> The current helper.h functions rely on hard coded assumptions about
>> target endianess to use the tswap macros. We also end up double
>> swapping a bunch of values if the target can run in multiple endianess
>> modes. Avoid this by getting the target to pass the endianess and size
>> via a MemOp and fixing up appropriately.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>    include/gdbstub/registers.h | 30 ++++++++++++++++++++++++++++++
>>    gdbstub/gdbstub.c           | 22 ++++++++++++++++++++++
>>    2 files changed, 52 insertions(+)
>>    create mode 100644 include/gdbstub/registers.h
>>
>> diff --git a/include/gdbstub/registers.h b/include/gdbstub/registers.h
>> new file mode 100644
>> index 0000000000..4abc7a6ae7
>> --- /dev/null
>> +++ b/include/gdbstub/registers.h
>> @@ -0,0 +1,30 @@
>> +/*
>> + * GDB Common Register Helpers
>> + *
>> + * Copyright (c) 2025 Linaro Ltd
>> + *
>> + * SPDX-License-Identifier: GPL-2.0-or-later
>> + */
>> +
>> +#ifndef GDB_REGISTERS_H
>> +#define GDB_REGISTERS_H
>> +
>> +#include "exec/memop.h"
>> +
>> +/**
>> + * gdb_get_register_value() - get register value for gdb
>> + * mo: size and endian MemOp
>> + * buf: GByteArray to store in target order
>> + * val: pointer to value in host order
>> + *
>> + * This replaces the previous legacy read functions with a single
>> + * function to handle all sizes. Passing @mo allows the target mode to
>> + * be taken into account and avoids using hard coded tswap() macros.
>> + *
>> + * Returns the number of bytes written to the array.
>> + */
>> +int gdb_get_register_value(MemOp op, GByteArray *buf, uint8_t *val);
>> +
>> +#endif /* GDB_REGISTERS_H */
>> +
>> +
>> diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
>> index 282e13e163..3d7b1028e4 100644
>> --- a/gdbstub/gdbstub.c
>> +++ b/gdbstub/gdbstub.c
>> @@ -32,6 +32,7 @@
>>    #include "exec/gdbstub.h"
>>    #include "gdbstub/commands.h"
>>    #include "gdbstub/syscalls.h"
>> +#include "gdbstub/registers.h"
>>    #ifdef CONFIG_USER_ONLY
>>    #include "accel/tcg/vcpu-state.h"
>>    #include "gdbstub/user.h"
>> @@ -45,6 +46,7 @@
>>    #include "system/runstate.h"
>>    #include "exec/replay-core.h"
>>    #include "exec/hwaddr.h"
>> +#include "exec/memop.h"
>>    
>>    #include "internals.h"
>>    
>> @@ -551,6 +553,26 @@ static int gdb_write_register(CPUState *cpu, uint8_t *mem_buf, int reg)
>>        return 0;
>>    }
>>    
>> +/*
>> + * Target helper function to read value into GByteArray, target
>> + * supplies the size and target endianess via the MemOp.
>> + */
>> +int gdb_get_register_value(MemOp op, GByteArray *buf, uint8_t *val)
>> +{
>> +    size_t bytes = memop_size(op);
>> +
>> +    if (op & MO_BSWAP) {
>> +        for ( int i = bytes ; i > 0; i--) {
>> +            g_byte_array_append(buf, &val[i - 1], 1);
>> +        };
>> +    } else {
>> +        g_byte_array_append(buf, val, bytes);
>> +    }
>> +
>> +    return bytes;
>> +}
>> +
>> +
>>    static void gdb_register_feature(CPUState *cpu, int base_reg,
>>                                     gdb_get_reg_cb get_reg, gdb_set_reg_cb set_reg,
>>                                     const GDBFeature *feature)
> 
> It could be preferable to set buf with the value, instead of simply
> appending the value. This way, there is no need to return the size, as
> it's contained in buffer size itself.
> 
> If we insist on returning the size, it's better to make it a parameter
> (and use a void parameter type), because at the moment, it gives the
> impression the function itself returns the value, which may be confusing.

Seems like it's the existing convention through 
gdb_set_reg_cb/gdb_get_reg_cb, so we have to follow this.