Re: [PATCH] plugins/stoptrigger: TCG plugin to stop execution under conditions

[Alex Bennée]

Simon Hamelin <simon.hamelin@grenoble-inp.org> writes:

> This new plugin allows to stop emulation using conditions on the
> emulation state. By setting this plugin arguments, it is possible
> to set an instruction count limit and/or trigger address(es) to stop at.
> The code returned at emulation exit can be customized.
>
> This plugin demonstrates how someone could stop QEMU execution.
> It could be used for research purposes to launch some code and
> deterministically stop it and understand where its execution flow went.
>
> Co-authored-by: Alexandre Iooss <erdnaxe@crans.org>
> Signed-off-by: Simon Hamelin <simon.hamelin@grenoble-inp.org>
> Signed-off-by: Alexandre Iooss <erdnaxe@crans.org>
> ---
>  contrib/plugins/Makefile      |   1 +
>  contrib/plugins/stoptrigger.c | 125 ++++++++++++++++++++++++++++++++++
>  docs/devel/tcg-plugins.rst    |  22 ++++++
>  3 files changed, 148 insertions(+)
>  create mode 100644 contrib/plugins/stoptrigger.c
>
> diff --git a/contrib/plugins/Makefile b/contrib/plugins/Makefile
> index 449ead1130..98a89d5c40 100644
> --- a/contrib/plugins/Makefile
> +++ b/contrib/plugins/Makefile
> @@ -28,6 +28,7 @@ NAMES += hwprofile
>  NAMES += cache
>  NAMES += drcov
>  NAMES += ips
> +NAMES += stoptrigger
>  
>  ifeq ($(CONFIG_WIN32),y)
>  SO_SUFFIX := .dll
> diff --git a/contrib/plugins/stoptrigger.c b/contrib/plugins/stoptrigger.c
> new file mode 100644
> index 0000000000..26b8a78f08
> --- /dev/null
> +++ b/contrib/plugins/stoptrigger.c
> @@ -0,0 +1,125 @@
> +/*
> + * Copyright (C) 2024, Simon Hamelin <simon.hamelin@grenoble-inp.org>
> + *
> + * Stop execution once a given address is reached or if the
> + * count of executed instructions reached a specified limit
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + */
> +
> +#include <assert.h>
> +#include <inttypes.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +
> +#include <qemu-plugin.h>
> +
> +QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
> +
> +static uint64_t icount;
> +static int icount_exit_code;
> +static uint64_t executed_instructions;
> +
> +static bool exit_on_icount;
> +static bool exit_on_address;
> +
> +/* Map trigger addresses to exit code */
> +static GHashTable *addrs_ht;
> +static GMutex addrs_ht_lock;
> +
> +static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
> +{
> +    executed_instructions++;
> +
> +    if (exit_on_icount && executed_instructions > icount) {
> +        /* We shouldn't execute more instructions than specified */
> +        g_assert(executed_instructions == icount + 1);
> +        qemu_plugin_outs("icount reached, exiting\n");
> +        exit(icount_exit_code);
> +    }

This would be trivially convertible to a scoreboard, with a counter
increment + conditional call back to helper which would be a lot more
efficient.

> +
> +    if (exit_on_address) {
> +        uint64_t insn_vaddr = GPOINTER_TO_UINT(udata);
> +        g_mutex_lock(&addrs_ht_lock);
> +        if (g_hash_table_contains(addrs_ht,
> GUINT_TO_POINTER(insn_vaddr))) {

We know the vaddr at runtime so couldn't we just plant the helper there?

> +            /* Exit triggered by address */
> +            int exit_code = GPOINTER_TO_INT(g_hash_table_lookup(addrs_ht,
> +                                            GUINT_TO_POINTER(insn_vaddr)));
> +            char *msg = g_strdup_printf("0x%" PRIx64 " reached, exiting\n",
> +                                        insn_vaddr);
> +            qemu_plugin_outs(msg);
> +            exit(exit_code);

I thing you should split the exit code into a helper so we can have a
common "do something" point for the stoptrigger.

> +        }
> +        g_mutex_unlock(&addrs_ht_lock);
> +    }
> +}
> +
> +static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
> +{
> +    /* Register vcpu_insn_exec callback on each instruction */
> +    size_t tb_n = qemu_plugin_tb_n_insns(tb);
> +    for (size_t i = 0; i < tb_n; i++) {
> +        struct qemu_plugin_insn *insn = qemu_plugin_tb_get_insn(tb, i);
> +        uint64_t insn_vaddr = qemu_plugin_insn_vaddr(insn);
> +        qemu_plugin_register_vcpu_insn_exec_cb(insn, vcpu_insn_exec,
> +                                               QEMU_PLUGIN_CB_NO_REGS,
> +                                               GUINT_TO_POINTER(insn_vaddr));
> +    }
> +}
> +
> +static void plugin_exit(qemu_plugin_id_t id, void *p)
> +{
> +    g_hash_table_destroy(addrs_ht);
> +}
> +
> +QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
> +                                           const qemu_info_t *info, int argc,
> +                                           char **argv)
> +{
> +    addrs_ht = g_hash_table_new(NULL, g_direct_equal);
> +
> +    for (int i = 0; i < argc; i++) {
> +        char *opt = argv[i];
> +        g_auto(GStrv) tokens = g_strsplit(opt, "=", 2);
> +        if (g_strcmp0(tokens[0], "icount") == 0) {
> +            g_auto(GStrv) icount_tokens = g_strsplit(tokens[1], ":", 2);
> +            icount = g_ascii_strtoull(icount_tokens[0], NULL, 0);
> +            if (icount < 1 || g_strrstr(icount_tokens[0], "-") != NULL) {
> +                fprintf(stderr, "icount parsing failed: '%s' must be a "
> +                        "positive integer\n", icount_tokens[0]);
> +                return -1;
> +            }
> +            if (icount_tokens[1]) {
> +                icount_exit_code = g_ascii_strtoull(icount_tokens[1], NULL, 
> 0);
> +            }
> +            exit_on_icount = true;
> +        } else if (g_strcmp0(tokens[0], "addr") == 0) {
> +            g_auto(GStrv) addr_tokens = g_strsplit(tokens[1], ":", 2);
> +            uint64_t exit_addr = g_ascii_strtoull(addr_tokens[0], NULL, 0);
> +            int exit_code = 0;
> +            if (addr_tokens[1]) {
> +                exit_code = g_ascii_strtoull(addr_tokens[1], NULL, 0);
> +            }
> +            g_mutex_lock(&addrs_ht_lock);
> +            g_hash_table_insert(addrs_ht, GUINT_TO_POINTER(exit_addr),
> +                                GINT_TO_POINTER(exit_code));
> +            g_mutex_unlock(&addrs_ht_lock);
> +            exit_on_address = true;
> +        } else {
> +            fprintf(stderr, "option parsing failed: %s\n", opt);
> +            return -1;
> +        }
> +    }
> +
> +    if (!exit_on_icount && !exit_on_address) {
> +        fprintf(stderr, "'icount' or 'addr' argument missing\n");
> +        return -1;
> +    }
> +
> +    /* Register translation block and exit callbacks */
> +    qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
> +    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
> +
> +    return 0;
> +}
> diff --git a/docs/devel/tcg-plugins.rst b/docs/devel/tcg-plugins.rst
> index f7d7b9e3a4..954623f9bf 100644
> --- a/docs/devel/tcg-plugins.rst
> +++ b/docs/devel/tcg-plugins.rst
> @@ -642,6 +642,28 @@ The plugin has a number of arguments, all of them are 
> optional:
>    configuration arguments implies ``l2=on``.
>    (default: N = 2097152 (2MB), B = 64, A = 16)
>  
> +- contrib/plugins/stoptrigger.c
> +
> +The stoptrigger plugin allows to setup triggers to stop emulation.
> +It can be used for research purposes to launch some code and precisely stop 
> it
> +and understand where its execution flow went.
> +
> +Two types of triggers can be configured: a count of instructions to stop at,
> +or an address to stop at. Multiple triggers can be set at once.
> +
> +By default, QEMU will exit with return code 0. A custom return code can be
> +configured for each trigger using ``:CODE`` syntax.

So I can see this being useful for general testing of "did the code get
to this point". However would it be worth considering other cases like
simply stopping the VM or triggering a gdbstub exception?

> +
> +For example, to stop at the 20-th instruction with return code 41, at address
> +0xd4 with return code 0 or at address 0xd8 with return code 42::
> +
> +  $ qemu-system-aarch64 $(QEMU_ARGS) \
> +    -plugin 
> ./contrib/plugins/libstoptrigger.so,icount=20:41,addr=0xd4,addr=0xd8:42 -d 
> plugin
> +
> +The plugin will log the reason of exit, for example::
> +
> +  0xd4 reached, exiting
> +
>  Plugin API
>  ==========

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro