qemu-monitor screendump very slow

qemu-monitor screendump often using 10-20% cpu usage of one core to take a small capture.

Most of the CPU usage seems to come from libpixman. There were many reports of libpixman becoming 8 times slower in newer releases.

https://github.com/qemu/qemu/blob/0c56c6ab68902281094c7aac6305e2321c34c187/ui/console.c#L285


Simple Valgrind Ir report.

--------------------------------------------------------------------------------
            Ir 
--------------------------------------------------------------------------------
56,592,286,124  PROGRAM TOTALS

--------------------------------------------------------------------------------
            Ir  file:function
--------------------------------------------------------------------------------
40,288,379,712  ???:0x000000000000caa0 [/usr/lib64/libpixman-1.so.0.34.0]
 3,585,795,168  ???:0x000000000006df20 [/usr/lib64/libpixman-1.so.0.34.0]
 1,763,982,432  ???:0x0000000000052360 [/usr/lib64/libpixman-1.so.0.34.0]
 1,517,832,033  ???:__memcpy_sse2_unaligned [/usr/lib64/libc-2.23.so]
   993,997,885  ???:__GI_mempcpy [/usr/lib64/libc-2.23.so]
   484,059,456  ???:0x0000000000050430 [/usr/lib64/libpixman-1.so.0.34.0]
   460,109,168  ???:pixman_image_composite32 [/usr/lib64/libpixman-1.so.0.34.0]

I tried taking a look on how to fix this, but it seems pixmap is deeply enrooted inside the monitor.  I wanted to try to simply take whats on the display and memcpy it into .ppm format manually creating the file header, but I cant figure out where the raw display buffer/image starts.

For example this is DisplaySurface:

struct DisplaySurface {
    pixman_format_code_t format;
    pixman_image_t *image;
    uint8_t flags;
#ifdef CONFIG_OPENGL
    GLenum glformat;
    GLenum gltype;
    GLuint texture;
#endif
};

Which as you can see already has the pixman_image_t. Maybe I should just work with that pixman_image_t?

The most effective solution IMO seems to just memcpy from the display into a premade header for a .ppm or .bmp file assuming 24 or 32 bpp. No need for libpixman.