[RFC PATCH 0/1] QEMU TCG plugin interface extensions

[RFC PATCH 0/1] QEMU TCG plugin interface extensions

[Florian Hauschild]

Hi all,

I extended the plugin interface with additional functionalities.
I wrote the extensions for fault injection/exploration reasearch using
QEMU. The additional functionalities for a plugin are:
  * Read and write guest memory
  * Read and write guest registers
  * Allow plugin to force QEMU into single step mode
  * Flush TB cache from plugin

Currently the changes are stored inside its own c file.
Should it be moved into one of the other plugin files?
Should a new config option be added to only enable the additional
extensions if set?

Best regards,
Florian


Florian Hauschild (1):
  QEMU plugin interface extension

 include/qemu/qemu-plugin.h   |  35 ++++++++++++
 plugins/meson.build          |   1 +
 plugins/readwriteextension.c | 106 +++++++++++++++++++++++++++++++++++
 3 files changed, 142 insertions(+)
 create mode 100644 plugins/readwriteextension.c

-- 
2.25.1

[RFC PATCH 1/1] QEMU plugin interface extension

[Florian Hauschild]

This extension covers functions:
  * to read and write guest memory
  * to read and write guest registers
  * to flush tb cache
  * to control single stepping of qemu from plugin

These changes allow the user to
  * collect more information about the behaviour of the system
  * change the guest state with a plugin during execution
  * control cache of tcg
  * allow for precise instrumentation in execution flow

Signed-off-by: Florian Hauschild <florian.hauschild@fs.ei.tum.de>
---
 include/qemu/qemu-plugin.h   |  35 ++++++++++++
 plugins/meson.build          |   1 +
 plugins/readwriteextension.c | 106 +++++++++++++++++++++++++++++++++++
 3 files changed, 142 insertions(+)
 create mode 100644 plugins/readwriteextension.c

diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
index e6e815abc5..c7a0c5f379 100644
--- a/include/qemu/qemu-plugin.h
+++ b/include/qemu/qemu-plugin.h
@@ -577,4 +577,39 @@ int qemu_plugin_n_max_vcpus(void);
  */
 void qemu_plugin_outs(const char *string);
 
+
+/**
+ * read_reg() read a register
+ * @reg: Number of the register
+ *
+ * Returns the value of the register
+ */
+uint64_t read_reg(int reg);
+
+/**
+ * write_reg() - write to a register
+ * @reg: number of the register
+ * @val: value written to register
+ */
+void write_reg(int reg, uint64_t val);
+
+/**
+ * plugin_flush_tb() - Flush the tb cache
+ */
+void plugin_flush_tb(void);
+
+/**
+ * plugin_rw_memory_cpu() - Function to read from and write to a guest address.
+ * @address: baseaddress of the memory section
+ * @buffer: buffer managed by caller the value should be written to
+ * @buf_size: size of the buffer and memory size read/written.
+ * @write: 1 if write, 0 if read
+ */
+int plugin_rw_memory_cpu(uint64_t address, uint8_t buffer[], size_t buf_size, char write);
+
+/**
+ * plugin_single_step() - Function to change single step behaviour from the plugin.
+ */
+void plugin_single_step(int enable);
+
 #endif /* QEMU_PLUGIN_API_H */
diff --git a/plugins/meson.build b/plugins/meson.build
index e77723010e..b95cbab0b1 100644
--- a/plugins/meson.build
+++ b/plugins/meson.build
@@ -10,4 +10,5 @@ specific_ss.add(when: 'CONFIG_PLUGIN', if_true: [files(
   'loader.c',
   'core.c',
   'api.c',
+  'readwriteextension.c',
 ), declare_dependency(link_args: plugin_ldflags)])
diff --git a/plugins/readwriteextension.c b/plugins/readwriteextension.c
new file mode 100644
index 0000000000..47460c396f
--- /dev/null
+++ b/plugins/readwriteextension.c
@@ -0,0 +1,106 @@
+/**
+ * QEMU Plugin read write extension code
+ *
+ * This is the code that allows the plugin to read and write
+ * memory and registers and flush the tb cache. Also allows
+ * to set QEMU into singlestep mode from Plugin.
+ *
+ * Based on plugin interface:
+ * Copyright (C) 2017, Emilio G. Cota <cota@braap.org>
+ * Copyright (C) 2019, Linaro
+ *
+ * Copyright (C) 2021 Florian Hauschild <florian.hauschild@tum.de>
+ *
+ * License: GNU GPL, version 2 or later.
+ *   See the COPYING file in the top-level directory.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+
+
+#include "qemu/osdep.h"
+#include "qemu/plugin.h"
+#include "hw/core/cpu.h"
+#include "cpu.h"
+#include "exec/exec-all.h"
+
+void plugin_async_flush_tb(CPUState *cpu, run_on_cpu_data arg);
+void plugin_async_flush_tb(CPUState *cpu, run_on_cpu_data arg)
+{
+    g_assert(cpu_in_exclusive_context(cpu));
+    tb_flush(cpu);
+}
+
+
+
+int plugin_rw_memory_cpu(uint64_t address, uint8_t buffer[], size_t buf_size, char write)
+{
+    return cpu_memory_rw_debug(current_cpu, address, buffer, buf_size, write);
+
+}
+
+
+void plugin_flush_tb(void)
+{
+    async_safe_run_on_cpu(current_cpu, plugin_async_flush_tb, RUN_ON_CPU_NULL);
+}
+
+static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
+{
+    CPUClass *cc = CPU_GET_CLASS(cpu);
+    if (reg < cc->gdb_num_core_regs) {
+        return cc->gdb_read_register(cpu, buf, reg);
+    }
+    return 0;
+}
+
+uint64_t read_reg(int reg)
+{
+    GByteArray *val = g_byte_array_new();
+    uint64_t reg_ret = 0;
+    int ret_bytes = plugin_read_register(current_cpu, val, reg);
+    if (ret_bytes == 1) {
+        reg_ret = val->data[0];
+    }
+    if (ret_bytes == 2) {
+        reg_ret = *(uint16_t *) &(val->data[0]);
+    }
+    if (ret_bytes == 4) {
+        reg_ret = *(uint32_t *) &(val->data[0]);
+    }
+    if (ret_bytes == 8) {
+        reg_ret = *(uint64_t *) &(val->data[0]);
+    }
+    return reg_ret;
+}
+
+void write_reg(int reg, uint64_t val)
+{
+    CPUState *cpu = current_cpu;
+    CPUClass *cc = CPU_GET_CLASS(cpu);
+
+    if (reg < cc->gdb_num_core_regs) {
+        cc->gdb_write_register(cpu, (uint8_t *) &val, reg);
+    }
+}
+
+void plugin_single_step(int enable)
+{
+    /* singlestep is set in softmmu/vl.c*/
+    static int orig_value;
+    static int executed = 1;
+
+    if (unlikely(executed == 1)) {
+        orig_value = singlestep;
+        executed = 2;
+    }
+
+    if (enable == 1) {
+        singlestep = 1;
+    } else {
+        singlestep = orig_value;
+    }
+
+    tb_flush(current_cpu);
+}
-- 
2.25.1

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Peter Maydell]

On Sat, 21 Aug 2021 at 10:48, Florian Hauschild
<florian.hauschild@fs.ei.tum.de> wrote:
>
> This extension covers functions:
>   * to read and write guest memory
>   * to read and write guest registers
>   * to flush tb cache
>   * to control single stepping of qemu from plugin
>
> These changes allow the user to
>   * collect more information about the behaviour of the system
>   * change the guest state with a plugin during execution
>   * control cache of tcg
>   * allow for precise instrumentation in execution flow

> +
> +static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
> +{
> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> +    if (reg < cc->gdb_num_core_regs) {
> +        return cc->gdb_read_register(cpu, buf, reg);
> +    }
> +    return 0;
> +}

At the point where these functions execute is the emulation
definitely stopped (ie no register values currently held
live in TCG locals) ?

-- PMM

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Florian Hauschild]

Am 21.08.21 um 15:18 schrieb Peter Maydell:
> On Sat, 21 Aug 2021 at 10:48, Florian Hauschild
> <florian.hauschild@fs.ei.tum.de> wrote:
>>
>> This extension covers functions:
>>   * to read and write guest memory
>>   * to read and write guest registers
>>   * to flush tb cache
>>   * to control single stepping of qemu from plugin
>>
>> These changes allow the user to
>>   * collect more information about the behaviour of the system
>>   * change the guest state with a plugin during execution
>>   * control cache of tcg
>>   * allow for precise instrumentation in execution flow
> 
>> +
>> +static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
>> +{
>> +    CPUClass *cc = CPU_GET_CLASS(cpu);
>> +    if (reg < cc->gdb_num_core_regs) {
>> +        return cc->gdb_read_register(cpu, buf, reg);
>> +    }
>> +    return 0;
>> +}
> 
> At the point where these functions execute is the emulation
> definitely stopped (ie no register values currently held
> live in TCG locals) ?
> 
> -- PMM
> 
I am not sure, if it is definitely stopped.
I call them during tb_exec_cb and insn_exec_cb.
I have used the extension on ARM and RISC-V single cpu guests and the
data collected is the one i would expect during normal execution on real
hardware. How this would behave on a multi cpu/core system i have not
tested yet.

Currently i am looking into this and as soon as i have found an answer i
will write back again.

Regards
Florian

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Peter Maydell]

On Tue, 24 Aug 2021 at 15:34, Florian Hauschild
<florian.hauschild@fs.ei.tum.de> wrote:
>
>
>
> Am 21.08.21 um 15:18 schrieb Peter Maydell:
> > On Sat, 21 Aug 2021 at 10:48, Florian Hauschild
> > <florian.hauschild@fs.ei.tum.de> wrote:
> >>
> >> This extension covers functions:
> >>   * to read and write guest memory
> >>   * to read and write guest registers
> >>   * to flush tb cache
> >>   * to control single stepping of qemu from plugin
> >>
> >> These changes allow the user to
> >>   * collect more information about the behaviour of the system
> >>   * change the guest state with a plugin during execution
> >>   * control cache of tcg
> >>   * allow for precise instrumentation in execution flow
> >
> >> +
> >> +static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
> >> +{
> >> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> >> +    if (reg < cc->gdb_num_core_regs) {
> >> +        return cc->gdb_read_register(cpu, buf, reg);
> >> +    }
> >> +    return 0;
> >> +}
> >
> > At the point where these functions execute is the emulation
> > definitely stopped (ie no register values currently held
> > live in TCG locals) ?

> I am not sure, if it is definitely stopped.
> I call them during tb_exec_cb and insn_exec_cb.
> I have used the extension on ARM and RISC-V single cpu guests and the
> data collected is the one i would expect during normal execution on real
> hardware. How this would behave on a multi cpu/core system i have not
> tested yet.

Multicore isn't relevant here. What you want to check for
is what happens when the TB covers multiple guest instructions
such that a later insn in the TB uses a register that is
set by an earlier insn in the TB, eg:

    mov x0, 0
    add x0, x0, 1
    add x0, x0, 1

In this case TCG is likely to generate code which does not
write back the intermediate 0 and 1 values of x0 to the CPUState
struct, and so reading x0 via the gdb_read_register interface
before the execution of the 3rd insn will continue to return
whatever value x0 had before execution of the TB started.

For the gdbstub's use of the gdb_read_register API, this
can't happen, because we always completely stop the CPU
(which means it is not inside a TB at all) before handling
gdbstub packets requesting register information.

I don't know whether the TCG plugin infrastructure takes steps
with its various callbacks to ensure that intermediate values
get written back to the CPU state before the callback is
invoked: it's possible that this is safe, or can be made to
be safe.

thanks
-- PMM

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Florian Hauschild]

Am 24.08.21 um 16:47 schrieb Peter Maydell:
> On Tue, 24 Aug 2021 at 15:34, Florian Hauschild
> <florian.hauschild@fs.ei.tum.de> wrote:
>>
>>
>>
>> Am 21.08.21 um 15:18 schrieb Peter Maydell:
>>> On Sat, 21 Aug 2021 at 10:48, Florian Hauschild
>>> <florian.hauschild@fs.ei.tum.de> wrote:
>>>>
>>>> This extension covers functions:
>>>>   * to read and write guest memory
>>>>   * to read and write guest registers
>>>>   * to flush tb cache
>>>>   * to control single stepping of qemu from plugin
>>>>
>>>> These changes allow the user to
>>>>   * collect more information about the behaviour of the system
>>>>   * change the guest state with a plugin during execution
>>>>   * control cache of tcg
>>>>   * allow for precise instrumentation in execution flow
>>>
>>>> +
>>>> +static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
>>>> +{
>>>> +    CPUClass *cc = CPU_GET_CLASS(cpu);
>>>> +    if (reg < cc->gdb_num_core_regs) {
>>>> +        return cc->gdb_read_register(cpu, buf, reg);
>>>> +    }
>>>> +    return 0;
>>>> +}
>>>
>>> At the point where these functions execute is the emulation
>>> definitely stopped (ie no register values currently held
>>> live in TCG locals) ?
> 
>> I am not sure, if it is definitely stopped.
>> I call them during tb_exec_cb and insn_exec_cb.
>> I have used the extension on ARM and RISC-V single cpu guests and the
>> data collected is the one i would expect during normal execution on real
>> hardware. How this would behave on a multi cpu/core system i have not
>> tested yet.
> 
> Multicore isn't relevant here. What you want to check for
> is what happens when the TB covers multiple guest instructions
> such that a later insn in the TB uses a register that is
> set by an earlier insn in the TB, eg:
> 
>     mov x0, 0
>     add x0, x0, 1
>     add x0, x0, 1
> 
> In this case TCG is likely to generate code which does not
> write back the intermediate 0 and 1 values of x0 to the CPUState
> struct, and so reading x0 via the gdb_read_register interface
> before the execution of the 3rd insn will continue to return
> whatever value x0 had before execution of the TB started.
> 
> For the gdbstub's use of the gdb_read_register API, this
> can't happen, because we always completely stop the CPU
> (which means it is not inside a TB at all) before handling
> gdbstub packets requesting register information.
> 
> I don't know whether the TCG plugin infrastructure takes steps
> with its various callbacks to ensure that intermediate values
> get written back to the CPU state before the callback is
> invoked: it's possible that this is safe, or can be made to
> be safe.
> 
> thanks
> -- PMM
> 

Sorry, i misunderstood your question.

Form my observation all three insn_cb would see x0 == 2. They are
executed at the end of a tb execution.

During my testing these changes were stable and i assume they are safe.
But thats why i chose RFC. I am new to QEMU and might overlook something
important.

Please correct me if i am wrong:
When the TB is executed, first the TB cb is executed, then the various
instruction cb. If you would like to see x0 in between instructions (e.g
mov and first add), QEMU need to be in single step mode.
The plugin infrastructure does have some sort of infrastructure to tell
the tcg if the registers are read or written to, but does apparently not
use it. The register values seem to be written back before the various
cbs are called.

Regards
Florian

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Peter Maydell]

On Thu, 26 Aug 2021 at 15:12, Florian Hauschild
<florian.hauschild@fs.ei.tum.de> wrote:
> Form my observation all three insn_cb would see x0 == 2. They are
> executed at the end of a tb execution.

The documentation for the insn_exec_cb says the cb is called
every time an instruction is executed. That won't always be at
the end of a TB, will it ?

> Please correct me if i am wrong:
> When the TB is executed, first the TB cb is executed, then the various
> instruction cb. If you would like to see x0 in between instructions (e.g
> mov and first add), QEMU need to be in single step mode.
> The plugin infrastructure does have some sort of infrastructure to tell
> the tcg if the registers are read or written to, but does apparently not
> use it. The register values seem to be written back before the various
> cbs are called.

Any new plugin API for "read/write registers" needs to work correctly
at any point where it is valid for it to be called, whether QEMU
is in single-step mode or not.

I guess we'll wait for Alex to get back from holiday and have a look at this...

thanks
-- PMM

Re: [RFC PATCH 1/1] QEMU plugin interface extension

[Alex Bennée]

Florian Hauschild <florian.hauschild@fs.ei.tum.de> writes:

> This extension covers functions:
>   * to read and write guest memory
>   * to read and write guest registers
>   * to flush tb cache
>   * to control single stepping of qemu from plugin

Next time please split the functionality into separate patches to aid
review.

>
> These changes allow the user to
>   * collect more information about the behaviour of the system

More introspection into guest state is a welcome improvement.

>   * change the guest state with a plugin during execution

I have no in principle objection to this but the wider community may
disagree. We are wary of plugins being used for GPL avoidance reasons.

>   * control cache of tcg

Why? From the plugins point of view the internal state of the translator
should be irrelevant. If it's not it's a bug.

>   * allow for precise instrumentation in execution flow

More precise than every instruction and memory access? How exactly?

>
> Signed-off-by: Florian Hauschild <florian.hauschild@fs.ei.tum.de>
> ---
>  include/qemu/qemu-plugin.h   |  35 ++++++++++++
>  plugins/meson.build          |   1 +
>  plugins/readwriteextension.c | 106 +++++++++++++++++++++++++++++++++++
>  3 files changed, 142 insertions(+)
>  create mode 100644 plugins/readwriteextension.c
>
> diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
> index e6e815abc5..c7a0c5f379 100644
> --- a/include/qemu/qemu-plugin.h
> +++ b/include/qemu/qemu-plugin.h
> @@ -577,4 +577,39 @@ int qemu_plugin_n_max_vcpus(void);
>   */
>  void qemu_plugin_outs(const char *string);
>  
> +
> +/**
> + * read_reg() read a register
> + * @reg: Number of the register
> + *
> + * Returns the value of the register
> + */
> +uint64_t read_reg(int reg);
> +
> +/**
> + * write_reg() - write to a register
> + * @reg: number of the register
> + * @val: value written to register
> + */
> +void write_reg(int reg, uint64_t val);
> +
> +/**
> + * plugin_flush_tb() - Flush the tb cache
> + */
> +void plugin_flush_tb(void);
> +
> +/**
> + * plugin_rw_memory_cpu() - Function to read from and write to a guest address.
> + * @address: baseaddress of the memory section
> + * @buffer: buffer managed by caller the value should be written to
> + * @buf_size: size of the buffer and memory size read/written.
> + * @write: 1 if write, 0 if read
> + */
> +int plugin_rw_memory_cpu(uint64_t address, uint8_t buffer[], size_t buf_size, char write);
> +
> +/**
> + * plugin_single_step() - Function to change single step behaviour from the plugin.
> + */
> +void plugin_single_step(int enable);
> +
>  #endif /* QEMU_PLUGIN_API_H */
> diff --git a/plugins/meson.build b/plugins/meson.build
> index e77723010e..b95cbab0b1 100644
> --- a/plugins/meson.build
> +++ b/plugins/meson.build
> @@ -10,4 +10,5 @@ specific_ss.add(when: 'CONFIG_PLUGIN', if_true: [files(
>    'loader.c',
>    'core.c',
>    'api.c',
> +  'readwriteextension.c',
>  ), declare_dependency(link_args: plugin_ldflags)])
> diff --git a/plugins/readwriteextension.c b/plugins/readwriteextension.c
> new file mode 100644
> index 0000000000..47460c396f
> --- /dev/null
> +++ b/plugins/readwriteextension.c
> @@ -0,0 +1,106 @@
> +/**
> + * QEMU Plugin read write extension code
> + *
> + * This is the code that allows the plugin to read and write
> + * memory and registers and flush the tb cache. Also allows
> + * to set QEMU into singlestep mode from Plugin.
> + *
> + * Based on plugin interface:
> + * Copyright (C) 2017, Emilio G. Cota <cota@braap.org>
> + * Copyright (C) 2019, Linaro
> + *
> + * Copyright (C) 2021 Florian Hauschild <florian.hauschild@tum.de>
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + *
> + * SPDX-License-Identifier: GPL-2.0-or-later
> + */
> +
> +
> +
> +#include "qemu/osdep.h"
> +#include "qemu/plugin.h"
> +#include "hw/core/cpu.h"
> +#include "cpu.h"
> +#include "exec/exec-all.h"
> +
> +void plugin_async_flush_tb(CPUState *cpu, run_on_cpu_data arg);
> +void plugin_async_flush_tb(CPUState *cpu, run_on_cpu_data arg)
> +{
> +    g_assert(cpu_in_exclusive_context(cpu));
> +    tb_flush(cpu);
> +}
> +
> +
> +
> +int plugin_rw_memory_cpu(uint64_t address, uint8_t buffer[], size_t buf_size, char write)
> +{
> +    return cpu_memory_rw_debug(current_cpu, address, buffer, buf_size, write);

Accessing memory during a plugin event is tricky. There is no way to
know if memory has changed underneath you. Would it instead be more
useful to derive the access from instrumented instructions? That way you
can know the exact value read or written at that exact moment in time.

> +}
> +
> +
> +void plugin_flush_tb(void)
> +{
> +    async_safe_run_on_cpu(current_cpu, plugin_async_flush_tb, RUN_ON_CPU_NULL);
> +}
> +
> +static int plugin_read_register(CPUState *cpu, GByteArray *buf, int reg)
> +{
> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> +    if (reg < cc->gdb_num_core_regs) {
> +        return cc->gdb_read_register(cpu, buf, reg);
> +    }
> +    return 0;
> +}

I'm not super keen on exposing gdb register id's to the plugins. For one
thing they can be dynamic and there is no way for the plugins to know
what the mapping is. I'd rather abstract the registers in a way that can
be cleanly used by HMP, logging and plugins.

> +
> +uint64_t read_reg(int reg)
> +{
> +    GByteArray *val = g_byte_array_new();
> +    uint64_t reg_ret = 0;
> +    int ret_bytes = plugin_read_register(current_cpu, val, reg);
> +    if (ret_bytes == 1) {
> +        reg_ret = val->data[0];
> +    }
> +    if (ret_bytes == 2) {
> +        reg_ret = *(uint16_t *) &(val->data[0]);
> +    }
> +    if (ret_bytes == 4) {
> +        reg_ret = *(uint32_t *) &(val->data[0]);
> +    }
> +    if (ret_bytes == 8) {
> +        reg_ret = *(uint64_t *) &(val->data[0]);
> +    }
> +    return reg_ret;
> +}

There are larger registers than 8 bytes - for example the various vector
implementations. They need to be accessible via this interface.

> +
> +void write_reg(int reg, uint64_t val)
> +{
> +    CPUState *cpu = current_cpu;
> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> +
> +    if (reg < cc->gdb_num_core_regs) {
> +        cc->gdb_write_register(cpu, (uint8_t *) &val, reg);
> +    }
> +}
> +
> +void plugin_single_step(int enable)
> +{
> +    /* singlestep is set in softmmu/vl.c*/
> +    static int orig_value;
> +    static int executed = 1;
> +
> +    if (unlikely(executed == 1)) {
> +        orig_value = singlestep;
> +        executed = 2;
> +    }
> +
> +    if (enable == 1) {
> +        singlestep = 1;
> +    } else {
> +        singlestep = orig_value;
> +    }
> +
> +    tb_flush(current_cpu);
> +}

Again why? What does this gain over instrumenting instructions themselves?

-- 
Alex Bennée

Re: [RFC PATCH 0/1] QEMU TCG plugin interface extensions

[Alexandre IOOSS]

[-- Attachment #1.1: Type: text/plain, Size: 1054 bytes --]

On 8/21/21 11:45 AM, Florian Hauschild wrote:
> Hi all,
> 
> I extended the plugin interface with additional functionalities.
> I wrote the extensions for fault injection/exploration reasearch using
> QEMU. The additional functionalities for a plugin are:
>    * Read and write guest memory
>    * Read and write guest registers
>    * Allow plugin to force QEMU into single step mode
>    * Flush TB cache from plugin

If something is added to read a register from a plugin, then execlog 
plugin could print the operands value of each instruction. This would 
definitely be helpful for side-channel analysis: the Hamming weight (sum 
of bits) of the last operand roughly models the power consumption 
side-channel leakage.

If I recall correctly, there are some concerns about allowing to access 
registers inside plugins. Past threads about reading/writing registers:
https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg08741.html
https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg04588.html

Thanks,
-- Alexandre


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