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Re: [PATCH 18/23] plugins: add an API to read registers

From: Akihiko Odaki
Subject: Re: [PATCH 18/23] plugins: add an API to read registers
Date: Thu, 22 Feb 2024 15:37:10 +0900
User-agent: Mozilla Thunderbird

On 2024/02/21 23:14, Alex Bennée wrote:
Akihiko Odaki <akihiko.odaki@daynix.com> writes:

On 2024/02/21 19:02, Alex Bennée wrote:
Akihiko Odaki <akihiko.odaki@daynix.com> writes:

On 2024/02/20 23:14, Alex Bennée wrote:
Akihiko Odaki <akihiko.odaki@daynix.com> writes:

On 2024/02/17 1:30, Alex Bennée wrote:
We can only request a list of registers once the vCPU has been
initialised so the user needs to use either call the get function on
vCPU initialisation or during the translation phase.
We don't expose the reg number to the plugin instead hiding it
an opaque handle. This allows for a bit of future proofing should the
internals need to be changed while also being hashed against the
CPUClass so we can handle different register sets per-vCPU in
hetrogenous situations.
Having an internal state within the plugins also allows us to expand
the interface in future (for example providing callbacks on register
change if the translator can track changes).
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1706
Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20240103173349.398526-39-alex.bennee@linaro.org>
Based-on: <20231025093128.33116-18-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
+ * Register handles
+ *
+ * The plugin infrastructure keeps hold of these internal data
+ * structures which are presented to plugins as opaque handles. They
+ * are global to the system and therefor additions to the hash table
+ * must be protected by the @reg_handle_lock.
+ *
+ * In order to future proof for up-coming heterogeneous work we want
+ * different entries for each CPU type while sharing them in the
+ * common case of multiple cores of the same type.
+ */
+static QemuMutex reg_handle_lock;
+struct qemu_plugin_register {
+    const char *name;
+    int gdb_reg_num;
+static GHashTable *reg_handles; /* hash table of PluginReg */
+/* Generate a stable key - would xxhash be overkill? */
+static gpointer cpu_plus_reg_to_key(CPUState *cs, int gdb_regnum)
+    uintptr_t key = (uintptr_t) cs->cc;
+    key ^= gdb_regnum;
+    return GUINT_TO_POINTER(key);

I have pointed out this is theoretically prone to collisions and
How is it unsafe? The aim is to share handles for the same CPUClass
rather than having a unique handle per register/cpu combo.

THe intention is legitimate, but the implementation is not safe. It
assumes (uintptr)cs->cc ^ gdb_regnum is unique, but there is no such
guarantee. The key of GHashTable must be unique; generating hashes of
keys should be done with hash_func given to g_hash_table_new().
This isn't a hash its a non-unique key. It is however unique for
the same register on the same class of CPU so for each vCPU in a system
can share the same opaque handles.
The hashing is done internally by glib. We would assert if there was
duplicate key referring to a different register.
I'm unsure what you want here? Do you have a suggestion for the key
generation algorithm? As the comment notes I did consider a more complex
mixing algorithm using xxhash but that wouldn't guarantee no clash

I suggest using a struct that holds both of cs->cc and gdb_regnum, and
pass g_direct_equal() and g_direct_hash() to g_hash_table_new().

We already do:

         if (!reg_handles) {
             reg_handles = g_hash_table_new(g_direct_hash, g_direct_equal);

But we can't use g_direct_equal with something that exceeds the width of
gpointer as it is a straight equality test of the key. What you are
suggesting requires allocating memory for each key and de-referencing
with a custom GEqualFunc.

My bad. I wrongly remembered g_direct_equal() and g_direct_hash(). It indeed seems to need a more complicated solution.

It is possible to write a GEqualFunc and a GHashFunc that consumes a struct but it is a chore. How about having a two-level GHashTable? reg_handles will be a GHashTable keyed with cs->cc, and another GHashTable will be keyed with gdb_regnum.

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