Re: [PATCH v2 40/43] contrib/plugins: extend execlog to track register changes

[Frédéric Pétrot]

Hello Alex,

  just reporting below what might be a riscv only oddity (also applies to
  patch 41 but easier to report here).

Le 03/01/2024 à 18:33, Alex Bennée a écrit :
> With the new plugin register API we can now track changes to register
> values. Currently the implementation is fairly dumb which will slow
> down if a large number of register values are being tracked. This
> could be improved by only instrumenting instructions which mention
> registers we are interested in tracking.
>
> Example usage:
>
>    ./qemu-aarch64 -D plugin.log -d plugin \
>       -cpu max,sve256=on \
>       -plugin contrib/plugins/libexeclog.so,reg=sp,reg=z\* \
>       ./tests/tcg/aarch64-linux-user/sha512-sve
>
> will display in the execlog any changes to the stack pointer (sp) and
> the SVE Z registers.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
> Based-On: <20231025093128.33116-19-akihiko.odaki@daynix.com>

> +static registers_init(int vcpu_index)
> +{
> +    GPtrArray *registers = g_ptr_array_new();
> +    g_autoptr(GArray) reg_list = qemu_plugin_get_registers(vcpu_index);
> +
> +    if (reg_list && reg_list->len) {
> +        /*
> +         * Go through each register in the complete list and
> +         * see if we want to track it.
> +         */
> +        for (int r = 0; r < reg_list->len; r++) {
> +            qemu_plugin_reg_descriptor *rd = &g_array_index(
> +                reg_list, qemu_plugin_reg_descriptor, r);

riscv csrs are not continously numbered and the dynamically generated gdb xml
seems to follow that scheme.
So the calls to Glib string functions output quite a few assertion
warnings because for the non existing csrs rd->name is NULL (and there
are a bit less than 4000 such cases for rv64g).
Checking for NULL and then continue is a simple way to solve the issue, but
I am not sure this is the proper way to proceed, as it might stand in the
generation of the riscv xml description for gdb.

Cheers,
Frédéric

> +            for (int p = 0; p < rmatches->len; p++) {
> +                g_autoptr(GPatternSpec) pat = 
> g_pattern_spec_new(rmatches->pdata[p]);
> +                if (g_pattern_match_string(pat, rd->name)) {
> +                    Register *reg = init_vcpu_register(vcpu_index, rd);
> +                    g_ptr_array_add(registers, reg);
> +                }
> +            }
> +        }
> +    }
> +    cpus[num_cpus].registers = registers;
> +}