Re: [PATCH] target/riscv: Use a direct cast for better performance

2023-10-10 Thread Richard Henderson 

On 10/7/23 02:02, Richard W.M. Jones wrote:

RISCV_CPU(cs) uses a checked cast.  When QOM cast debugging is enabled
this adds about 5% total overhead when emulating RV64 on x86-64 host.

Using a RISC-V guest with 16 vCPUs, 16 GB of guest RAM, virtio-blk
disk.  The guest has a copy of the qemu source tree.  The test
involves compiling the qemu source tree with 'make clean; time make -j16'.

Before making this change the compile step took 449 & 447 seconds over
two consecutive runs.

After making this change, 428 & 422 seconds.

The saving is about 5%.

Thanks: Paolo Bonzini
Signed-off-by: Richard W.M. Jones 
---
  target/riscv/cpu_helper.c | 6 +-
  1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 3a02079290..6174d99fb2 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -66,7 +66,11 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
uint64_t *cs_base, uint32_t *pflags)
  {
  CPUState *cs = env_cpu(env);
-RISCVCPU *cpu = RISCV_CPU(cs);
+/*
+ * Using the checked cast RISCV_CPU(cs) imposes ~ 5% overhead when
+ * qemu cast debugging is enabled, so use a direct cast instead.
+ */
+RISCVCPU *cpu = (RISCVCPU *)cs;


RISCVCPU *cpu = env_archcpu(env);

and avoid "CPUState *cs" entirely.


r~

Re: [PATCH] target/riscv: Use a direct cast for better performance

2023-10-07 Thread Daniel Henrique Barboza 




On 10/7/23 06:02, Richard W.M. Jones wrote:

RISCV_CPU(cs) uses a checked cast.  When QOM cast debugging is enabled
this adds about 5% total overhead when emulating RV64 on x86-64 host.

Using a RISC-V guest with 16 vCPUs, 16 GB of guest RAM, virtio-blk
disk.  The guest has a copy of the qemu source tree.  The test
involves compiling the qemu source tree with 'make clean; time make -j16'.

Before making this change the compile step took 449 & 447 seconds over
two consecutive runs.

After making this change, 428 & 422 seconds.

The saving is about 5%.

Thanks: Paolo Bonzini
Signed-off-by: Richard W.M. Jones 
---


Reviewed-by: Daniel Henrique Barboza 



  target/riscv/cpu_helper.c | 6 +-
  1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 3a02079290..6174d99fb2 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -66,7 +66,11 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
uint64_t *cs_base, uint32_t *pflags)
  {
  CPUState *cs = env_cpu(env);
-RISCVCPU *cpu = RISCV_CPU(cs);
+/*
+ * Using the checked cast RISCV_CPU(cs) imposes ~ 5% overhead when
+ * qemu cast debugging is enabled, so use a direct cast instead.
+ */
+RISCVCPU *cpu = (RISCVCPU *)cs;
  RISCVExtStatus fs, vs;
  uint32_t flags = 0;

Re: [PATCH] target/riscv: Use a direct cast for better performance

2023-10-07 Thread Richard W.M. Jones 
If you're interested in how I found this problem, it was done using
'perf report -a -g' & flamegraphs.  This is the flamegraph of qemu (on
the host) when the guest is running the parallel compile:

  http://oirase.annexia.org/tmp/qemu-riscv.svg

If you click into 'CPU_0/TCG' at the bottom left (all the vCPUs
basically act alike), and then go to 'cpu_get_tb_cpu_state' you can
see the call to 'object_dynamic_cast_assert' taking considerable time.

If you zoom out, hit Ctrl F and type 'object_dynamic_cast_assert' into
the search box then the flamegraph will tell you this call takes about
6.6% of total time (not all, but most, attributable to the call from
'cpu_get_tb_cpu_state' -> 'object_dynamic_cast_assert').

There are several other issues in the flamegraph which I'm trying to
address, but this was the simplest one.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html

[PATCH] target/riscv: Use a direct cast for better performance

2023-10-07 Thread Richard W.M. Jones 
RISCV_CPU(cs) uses a checked cast.  When QOM cast debugging is enabled
this adds about 5% total overhead when emulating RV64 on x86-64 host.

Using a RISC-V guest with 16 vCPUs, 16 GB of guest RAM, virtio-blk
disk.  The guest has a copy of the qemu source tree.  The test
involves compiling the qemu source tree with 'make clean; time make -j16'.

Before making this change the compile step took 449 & 447 seconds over
two consecutive runs.

After making this change, 428 & 422 seconds.

The saving is about 5%.

Thanks: Paolo Bonzini
Signed-off-by: Richard W.M. Jones 
---
 target/riscv/cpu_helper.c | 6 +-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 3a02079290..6174d99fb2 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -66,7 +66,11 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
   uint64_t *cs_base, uint32_t *pflags)
 {
 CPUState *cs = env_cpu(env);
-RISCVCPU *cpu = RISCV_CPU(cs);
+/*
+ * Using the checked cast RISCV_CPU(cs) imposes ~ 5% overhead when
+ * qemu cast debugging is enabled, so use a direct cast instead.
+ */
+RISCVCPU *cpu = (RISCVCPU *)cs;
 RISCVExtStatus fs, vs;
 uint32_t flags = 0;
 
-- 
2.41.0