Re: [Qemu-devel] [PATCH v3 1/6] hypertrace: Add documentation

[Daniel P. Berrange]

On Wed, Sep 28, 2016 at 03:05:36PM +0200, Lluís Vilanova wrote:
> Signed-off-by: Lluís Vilanova <address@hidden>
> ---
>  docs/hypertrace.txt |  232 
> +++++++++++++++++++++++++++++++++++++++++++++++++++
>  docs/tracing.txt    |    3 +
>  2 files changed, 235 insertions(+)
>  create mode 100644 docs/hypertrace.txt

> +== Quick guide ==
> +
> +This shows an example of using the hypertrace channel to trace the guest 
> memory
> +accesses only in a specific guest code region, which is identified by calls 
> to
> +the hypertrace channel.
> +
> +We are going to trace memory accesses to disk using QEMU's "log" backend, and
> +will use QEMU's "dtrace" (SystemTap) backend to ensure memory accesses are 
> only
> +traced in the guest code region of interest. The first time the guest code
> +invokes the hypertrace channel, we will start tracing the
> +"guest_mem_before_exec" event using dtrace, and then will disable it the 
> second
> +time around.
> +
> +Tracing is done with "log" because it is more efficient than using "dtrace" 
> in
> +high-volume events like memory accesses. If the event rate is low, you can
> +ignore the "log" backend and simply use "dtrace"'s printf, while still using 
> the
> +hypertrace events to identify guest semantics.
> +
> +1. Set the tracing backends and number of arguments for the hypertrace 
> events:
> +
> +    mkdir /tmp/qemu-build
> +    cd /tmp/qemu-build
> +    /path/to/qemu-source/configure              \
> +        --enable-trace-backends=dtrace,log      \
> +        --with-hypertrace-args=4                \
> +        --prefix=/tmp/qemu-install
> +    make -j install
> +
> +2. Enable the event "guest_mem_before_exec":
> +
> +    sed -i -e 's/disable vcpu tcg guest_mem_before/vcpu tcg 
> guest_mem_before/g' /path/to/qemu-source/trace-events
> +
> +3. Compile the guest support code:
> +
> +    make -C /tmp/qemu-build/x86_64-linux-user/hypertrace/guest/user
> +    make -C /tmp/qemu-build/x86_64-softmmu/hypertrace/guest/user
> +    make -C /tmp/qemu-build/x86_64-softmmu/hypertrace/guest/linux-module
> +
> +   If you need to cross-compile the guest library, set the 'CC' variable:
> +
> +    make -C /tmp/qemu-build/mipsel-linux-user/hypertrace/guest/user 
> CC=mipsel-gnu-linux-gcc
> +
> +3. Create a guest application using "qemu-hypertrace.h" to interact with the
> +   hypertrace channel:
> +
> +    cat > /tmp/my-hypertrace.c <<\EOF
> +    #include <stdio.h>
> +    #include <errno.h>
> +    #include <stdlib.h>
> +    #include <string.h>
> +    #include <qemu-hypertrace.h>
> +    
> +    
> +    int main(int argc, char **argv)
> +    {
> +        char *base = NULL;
> +        if (argc > 1) {
> +            base = argv[1];
> +        }
> +    
> +        /* In 'user' mode this path must be the same we will use to start 
> QEMU. */
> +        if (qemu_hypertrace_init(base) != 0) {
> +            perror("error: qemu_hypertrace_init");
> +            abort();
> +        }
> +    
> +        /* Set additional event arguments */
> +        uint64_t client  = 0;

IIUC, 'client' is the field that the host uses to distinguish
between different guest applications or different guest threads.

How are applications expected to assign themselves a unique
value for this field ? 

It feels that life would be easier if you made this field be
128-bits instead of 64-bit, as then you could encode a UUID
in it, making it much easier to guarnatee uniqueness without
having to have a central authority.

> +        uint64_t *data = qemu_hypertrace_data(client);
> +        data[0] = 0xcafe;
> +        data[1] = 0xdead;
> +        data[2] = 0xbeef;
> +    
> +        /* Emit event to start tracing */
> +        qemu_hypertrace(client, 1);
> +
> +        /* Computation in between */
> +        printf("Some computation...\n");
> +
> +        /* Emit event to stop tracing */
> +        qemu_hypertrace(client, 0);
> +    }
> +    EOF
> +
> +    gcc -o /tmp/my-hypertrace-user /tmp/my-hypertrace.c                      
>               \
> +        
> /tmp/qemu-build/x86_64-linux-user/hypertrace/guest/user/libqemu-hypertrace-guest.a
>  \
> +        -I/tmp/qemu-install/include -lpthread
> +
> +    gcc -o /tmp/my-hypertrace-softmmu /tmp/my-hypertrace.c                   
>            \
> +        
> /tmp/qemu-build/x86_64-softmmu/hypertrace/guest/user/libqemu-hypertrace-guest.a
>  \
> +        -I/tmp/qemu-install/include -lpthread

Regards,
Daniel
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