[PATCH v2 1/2] tcg: Document tb_lookup() and tcg_tb_lookup()

[PATCH v2 1/2] tcg: Document tb_lookup() and tcg_tb_lookup()

[Ilya Leoshkevich]

These similarly named functions serve different purposes; add
docstrings to highlight them.

Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 accel/tcg/cpu-exec.c | 15 ++++++++++++++-
 include/tcg/tcg.h    | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index d48b82a9325..8b773d88478 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -249,7 +249,20 @@ static TranslationBlock *tb_htable_lookup(CPUState *cpu, vaddr pc,
     return qht_lookup_custom(&tb_ctx.htable, &desc, h, tb_lookup_cmp);
 }
 
-/* Might cause an exception, so have a longjmp destination ready */
+/**
+ * tb_lookup:
+ * @cpu: CPU that will execute the returned translation block
+ * @pc: guest PC
+ * @cs_base: arch-specific value associated with translation block
+ * @flags: arch-specific translation block flags
+ * @cflags: CF_* flags
+ *
+ * Look up a translation block inside the QHT using @pc, @cs_base, @flags and
+ * @cflags. Uses @cpu's tb_jmp_cache. Might cause an exception, so have a
+ * longjmp destination ready.
+ *
+ * Returns: an existing translation block or NULL.
+ */
 static inline TranslationBlock *tb_lookup(CPUState *cpu, vaddr pc,
                                           uint64_t cs_base, uint32_t flags,
                                           uint32_t cflags)
diff --git a/include/tcg/tcg.h b/include/tcg/tcg.h
index a77ed12b9dc..057df6c4599 100644
--- a/include/tcg/tcg.h
+++ b/include/tcg/tcg.h
@@ -746,10 +746,51 @@ void tcg_region_reset_all(void);
 size_t tcg_code_size(void);
 size_t tcg_code_capacity(void);
 
+/**
+ * tcg_tb_insert:
+ * @tb: translation block to insert
+ *
+ * Insert @tb into the region trees.
+ */
 void tcg_tb_insert(TranslationBlock *tb);
+
+/**
+ * tcg_tb_remove:
+ * @tb: translation block to remove
+ *
+ * Remove @tb from the region trees.
+ */
 void tcg_tb_remove(TranslationBlock *tb);
+
+/**
+ * tcg_tb_lookup:
+ * @tc_ptr: host PC to look up
+ *
+ * Look up a translation block inside the region trees by @tc_ptr. This is
+ * useful for exception handling, but must not be used for the purposes of
+ * executing the returned translation block. See struct tb_tc for more
+ * information.
+ *
+ * Returns: a translation block previously inserted into the region trees,
+ * such that @tc_ptr points anywhere inside the code generated for it, or
+ * NULL.
+ */
 TranslationBlock *tcg_tb_lookup(uintptr_t tc_ptr);
+
+/**
+ * tcg_tb_foreach:
+ * @func: callback
+ * @user_data: opaque value to pass to @callback
+ *
+ * Call @func for each translation block inserted into the region trees.
+ */
 void tcg_tb_foreach(GTraverseFunc func, gpointer user_data);
+
+/**
+ * tcg_nb_tbs:
+ *
+ * Returns: the number of translation blocks inserted into the region trees.
+ */
 size_t tcg_nb_tbs(void);
 
 /* user-mode: Called with mmap_lock held.  */
-- 
2.47.1

[PATCH v2 2/2] accel/tcg: Call tcg_tb_insert() for one-insn TBs

[Ilya Leoshkevich]

Currently one-insn TBs created from I/O memory are not added to
region_trees. Therefore, when they generate exceptions, they are not
handled by cpu_restore_state_from_tb().

For x86 this is not a problem, because x86_restore_state_to_opc() only
restores pc and cc, which already have the correct values if the first
TB instruction causes an exception. However, on several other
architectures, restore_state_to_opc() is not stricly limited to state
restoration and affects some exception-related registers, where guests
can notice incorrect values, for example:

- arm's exception.syndrome;
- hppa's unwind_breg;
- riscv's excp_uw2;
- s390x's int_pgm_ilen.

Fix by always calling tcg_tb_insert(). This may increase the size of
region_trees, but tcg_region_reset_all() clears it once code_gen_buffer
fills up, so it will not grow uncontrollably.

Do not call tb_link_page(), which would add such TBs to the QHT, to
prevent tb_lookup() from finding them. These TBs are single-use, since
subsequent reads from I/O memory may return different values; they are
not removed from code_gen_buffer only in order to keep things simple.

Co-developed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 accel/tcg/translate-all.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index 453eb20ec95..7ec1c53f240 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -531,23 +531,32 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
         tb_reset_jump(tb, 1);
     }
 
+    /*
+     * Insert TB into the corresponding region tree before publishing it
+     * through QHT. Otherwise rewinding happened in the TB might fail to
+     * lookup itself using host PC.
+     */
+    tcg_tb_insert(tb);
+
     /*
      * If the TB is not associated with a physical RAM page then it must be
-     * a temporary one-insn TB, and we have nothing left to do. Return early
-     * before attempting to link to other TBs or add to the lookup table.
+     * a temporary one-insn TB.
+     *
+     * Such TBs must be added to region trees in order to make sure that
+     * restore_state_to_opc() - which on some architectures is not limited to
+     * rewinding, but also affects exception handling! - is called when such a
+     * TB causes an exception.
+     *
+     * At the same time, temporary one-insn TBs must be executed at most once,
+     * because subsequent reads from, e.g., I/O memory may return different
+     * values. So return early before attempting to link to other TBs or add
+     * to the QHT.
      */
     if (tb_page_addr0(tb) == -1) {
         assert_no_pages_locked();
         return tb;
     }
 
-    /*
-     * Insert TB into the corresponding region tree before publishing it
-     * through QHT. Otherwise rewinding happened in the TB might fail to
-     * lookup itself using host PC.
-     */
-    tcg_tb_insert(tb);
-
     /*
      * No explicit memory barrier is required -- tb_link_page() makes the
      * TB visible in a consistent state.
-- 
2.47.1

Re: [PATCH v2 1/2] tcg: Document tb_lookup() and tcg_tb_lookup()

[Richard Henderson]

On 1/16/25 13:31, Ilya Leoshkevich wrote:
> These similarly named functions serve different purposes; add
> docstrings to highlight them.
> 
> Suggested-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   accel/tcg/cpu-exec.c | 15 ++++++++++++++-
>   include/tcg/tcg.h    | 41 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 55 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~

> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index d48b82a9325..8b773d88478 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -249,7 +249,20 @@ static TranslationBlock *tb_htable_lookup(CPUState *cpu, vaddr pc,
>       return qht_lookup_custom(&tb_ctx.htable, &desc, h, tb_lookup_cmp);
>   }
>   
> -/* Might cause an exception, so have a longjmp destination ready */
> +/**
> + * tb_lookup:
> + * @cpu: CPU that will execute the returned translation block
> + * @pc: guest PC
> + * @cs_base: arch-specific value associated with translation block
> + * @flags: arch-specific translation block flags
> + * @cflags: CF_* flags
> + *
> + * Look up a translation block inside the QHT using @pc, @cs_base, @flags and
> + * @cflags. Uses @cpu's tb_jmp_cache. Might cause an exception, so have a
> + * longjmp destination ready.
> + *
> + * Returns: an existing translation block or NULL.
> + */
>   static inline TranslationBlock *tb_lookup(CPUState *cpu, vaddr pc,
>                                             uint64_t cs_base, uint32_t flags,
>                                             uint32_t cflags)
> diff --git a/include/tcg/tcg.h b/include/tcg/tcg.h
> index a77ed12b9dc..057df6c4599 100644
> --- a/include/tcg/tcg.h
> +++ b/include/tcg/tcg.h
> @@ -746,10 +746,51 @@ void tcg_region_reset_all(void);
>   size_t tcg_code_size(void);
>   size_t tcg_code_capacity(void);
>   
> +/**
> + * tcg_tb_insert:
> + * @tb: translation block to insert
> + *
> + * Insert @tb into the region trees.
> + */
>   void tcg_tb_insert(TranslationBlock *tb);
> +
> +/**
> + * tcg_tb_remove:
> + * @tb: translation block to remove
> + *
> + * Remove @tb from the region trees.
> + */
>   void tcg_tb_remove(TranslationBlock *tb);
> +
> +/**
> + * tcg_tb_lookup:
> + * @tc_ptr: host PC to look up
> + *
> + * Look up a translation block inside the region trees by @tc_ptr. This is
> + * useful for exception handling, but must not be used for the purposes of
> + * executing the returned translation block. See struct tb_tc for more
> + * information.
> + *
> + * Returns: a translation block previously inserted into the region trees,
> + * such that @tc_ptr points anywhere inside the code generated for it, or
> + * NULL.
> + */
>   TranslationBlock *tcg_tb_lookup(uintptr_t tc_ptr);
> +
> +/**
> + * tcg_tb_foreach:
> + * @func: callback
> + * @user_data: opaque value to pass to @callback
> + *
> + * Call @func for each translation block inserted into the region trees.
> + */
>   void tcg_tb_foreach(GTraverseFunc func, gpointer user_data);
> +
> +/**
> + * tcg_nb_tbs:
> + *
> + * Returns: the number of translation blocks inserted into the region trees.
> + */
>   size_t tcg_nb_tbs(void);
>   
>   /* user-mode: Called with mmap_lock held.  */

Re: [PATCH v2 1/2] tcg: Document tb_lookup() and tcg_tb_lookup()

[Alex Bennée]

Ilya Leoshkevich <iii@linux.ibm.com> writes:

> These similarly named functions serve different purposes; add
> docstrings to highlight them.
>
> Suggested-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro

Re: [PATCH v2 2/2] accel/tcg: Call tcg_tb_insert() for one-insn TBs

[Alex Bennée]

Ilya Leoshkevich <iii@linux.ibm.com> writes:

> Currently one-insn TBs created from I/O memory are not added to
> region_trees. Therefore, when they generate exceptions, they are not
> handled by cpu_restore_state_from_tb().
>
> For x86 this is not a problem, because x86_restore_state_to_opc() only
> restores pc and cc, which already have the correct values if the first
> TB instruction causes an exception. However, on several other
> architectures, restore_state_to_opc() is not stricly limited to state
> restoration and affects some exception-related registers, where guests
> can notice incorrect values, for example:
>
> - arm's exception.syndrome;
> - hppa's unwind_breg;
> - riscv's excp_uw2;
> - s390x's int_pgm_ilen.
>
> Fix by always calling tcg_tb_insert(). This may increase the size of
> region_trees, but tcg_region_reset_all() clears it once code_gen_buffer
> fills up, so it will not grow uncontrollably.
>
> Do not call tb_link_page(), which would add such TBs to the QHT, to
> prevent tb_lookup() from finding them. These TBs are single-use, since
> subsequent reads from I/O memory may return different values; they are
> not removed from code_gen_buffer only in order to keep things simple.
>
> Co-developed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro

Re: [PATCH v2 1/2] tcg: Document tb_lookup() and tcg_tb_lookup()

[Richard Henderson]

On 1/16/25 13:31, Ilya Leoshkevich wrote:
> These similarly named functions serve different purposes; add
> docstrings to highlight them.
> 
> Suggested-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   accel/tcg/cpu-exec.c | 15 ++++++++++++++-
>   include/tcg/tcg.h    | 41 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 55 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

And queued, thanks.


r~

> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index d48b82a9325..8b773d88478 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -249,7 +249,20 @@ static TranslationBlock *tb_htable_lookup(CPUState *cpu, vaddr pc,
>       return qht_lookup_custom(&tb_ctx.htable, &desc, h, tb_lookup_cmp);
>   }
>   
> -/* Might cause an exception, so have a longjmp destination ready */
> +/**
> + * tb_lookup:
> + * @cpu: CPU that will execute the returned translation block
> + * @pc: guest PC
> + * @cs_base: arch-specific value associated with translation block
> + * @flags: arch-specific translation block flags
> + * @cflags: CF_* flags
> + *
> + * Look up a translation block inside the QHT using @pc, @cs_base, @flags and
> + * @cflags. Uses @cpu's tb_jmp_cache. Might cause an exception, so have a
> + * longjmp destination ready.
> + *
> + * Returns: an existing translation block or NULL.
> + */
>   static inline TranslationBlock *tb_lookup(CPUState *cpu, vaddr pc,
>                                             uint64_t cs_base, uint32_t flags,
>                                             uint32_t cflags)
> diff --git a/include/tcg/tcg.h b/include/tcg/tcg.h
> index a77ed12b9dc..057df6c4599 100644
> --- a/include/tcg/tcg.h
> +++ b/include/tcg/tcg.h
> @@ -746,10 +746,51 @@ void tcg_region_reset_all(void);
>   size_t tcg_code_size(void);
>   size_t tcg_code_capacity(void);
>   
> +/**
> + * tcg_tb_insert:
> + * @tb: translation block to insert
> + *
> + * Insert @tb into the region trees.
> + */
>   void tcg_tb_insert(TranslationBlock *tb);
> +
> +/**
> + * tcg_tb_remove:
> + * @tb: translation block to remove
> + *
> + * Remove @tb from the region trees.
> + */
>   void tcg_tb_remove(TranslationBlock *tb);
> +
> +/**
> + * tcg_tb_lookup:
> + * @tc_ptr: host PC to look up
> + *
> + * Look up a translation block inside the region trees by @tc_ptr. This is
> + * useful for exception handling, but must not be used for the purposes of
> + * executing the returned translation block. See struct tb_tc for more
> + * information.
> + *
> + * Returns: a translation block previously inserted into the region trees,
> + * such that @tc_ptr points anywhere inside the code generated for it, or
> + * NULL.
> + */
>   TranslationBlock *tcg_tb_lookup(uintptr_t tc_ptr);
> +
> +/**
> + * tcg_tb_foreach:
> + * @func: callback
> + * @user_data: opaque value to pass to @callback
> + *
> + * Call @func for each translation block inserted into the region trees.
> + */
>   void tcg_tb_foreach(GTraverseFunc func, gpointer user_data);
> +
> +/**
> + * tcg_nb_tbs:
> + *
> + * Returns: the number of translation blocks inserted into the region trees.
> + */
>   size_t tcg_nb_tbs(void);
>   
>   /* user-mode: Called with mmap_lock held.  */