Re: [Qemu-devel] [PATCH] Introduce qemu_madvise()

[Alexander Graf]

On 11.09.2010, at 19:05, Andreas Färber wrote:

> From: Andreas Färber <address@hidden>
> 
> vl.c has a Sun-specific hack to supply a prototype for madvise(),
> but the call site has apparently moved to arch_init.c.
> The underlying issue is that madvise() is not a POSIX function,
> therefore Solaris' _POSIX_C_SOURCE suppresses the prototype.
> 
> Haiku doesn't implement madvise() at all.
> OpenBSD however doesn't implement posix_madvise().
> 
> Check for posix_madvise() in configure and supply qemu_madvise() as wrapper.
> Use qemu_madvise() in arch_init.c's ram_load().
> 
> http://www.opengroup.org/onlinepubs/9699919799/functions/posix_madvise.html
> 
> Remaining madvise() users:
> exec.c: limited to __linux__ and/or MADV_MERGEABLE (no POSIX equivalent)
> kvm-all.c: limited to MADV_DONTFORK (no POSIX equivalent),
>           otherwise runtime error if !kvm_has_sync_mmu()
> hw/virtio-balloon.c: limited to __linux__
> 
> v1 -> v2:
> * Don't rely on posix_madvise() availability, add qemu_madvise().
>  Suggested by Blue Swirl.
> 
> Signed-off-by: Andreas Färber <address@hidden>
> Cc: Blue Swirl <address@hidden>
> ---
> arch_init.c |    2 +-
> configure   |   11 +++++++++++
> osdep.c     |   26 ++++++++++++++++++++++++++
> osdep.h     |    4 ++++
> vl.c        |    3 ---
> 5 files changed, 42 insertions(+), 4 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index e468c0c..a910033 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -396,7 +396,7 @@ int ram_load(QEMUFile *f, void *opaque, int version_id)
> #ifndef _WIN32
>             if (ch == 0 &&
>                 (!kvm_enabled() || kvm_has_sync_mmu())) {
> -                madvise(host, TARGET_PAGE_SIZE, MADV_DONTNEED);
> +                qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);

Is this the only occurence in the whole code base? This patch should convert 
all users, right?

>             }
> #endif
>         } else if (flags & RAM_SAVE_FLAG_PAGE) {
> diff --git a/configure b/configure
> index 4061cb7..5e6f3e1 100755
> --- a/configure
> +++ b/configure
> @@ -2069,6 +2069,17 @@ if compile_prog "" "" ; then
> fi
> 
> ##########################################
> +# check if we have posix_madvise
> +
> +cat > $TMPC << EOF
> +#include <sys/mman.h>
> +int main(void) { posix_madvise(NULL, 0, POSIX_MADV_DONTNEED); return 0; }
> +EOF
> +if compile_prog "" "" ; then
> +    QEMU_CFLAGS="-DCONFIG_POSIX_MADVISE ${QEMU_CFLAGS}"
> +fi
> +
> +##########################################
> # check if trace backend exists
> 
> sh "$source_path/tracetool" "--$trace_backend" --check-backend > /dev/null 2> 
> /dev/null
> diff --git a/osdep.c b/osdep.c
> index 30426ff..8c09597 100644
> --- a/osdep.c
> +++ b/osdep.c
> @@ -28,6 +28,7 @@
> #include <errno.h>
> #include <unistd.h>
> #include <fcntl.h>
> +#include <sys/mman.h>
> 
> /* Needed early for CONFIG_BSD etc. */
> #include "config-host.h"
> @@ -139,6 +140,31 @@ void qemu_vfree(void *ptr)
> 
> #endif
> 
> +int qemu_madvise(void *addr, size_t len, int advice)
> +{
> +#ifdef CONFIG_POSIX_MADVISE
> +    switch (advice) {
> +        case QEMU_MADV_DONTNEED:
> +            advice = POSIX_MADV_DONTNEED;
> +            break;
> +        default:
> +            fprintf(stderr, "Advice %d unhandled.\n", advice);

Advise :) It should probably also be 'madvise' here. Plus, I'd recommend %x as 
that makes the bits slightly more obvious.

> +            abort();
> +    }
> +    return posix_madvise(addr, len, advice);
> +#else
> +    switch (advice) {
> +        case QEMU_MADV_DONTNEED:
> +            advice = MADV_DONTNEED;
> +            break;
> +        default:
> +            fprintf(stderr, "Advice %d unhandled.\n", advice);
> +            abort();
> +    }
> +    return madvise(addr, len, advice);

So what do you do on haiku where there's no madvise?

> +#endif
> +}
> +
> int qemu_create_pidfile(const char *filename)
> {
>     char buffer[128];
> diff --git a/osdep.h b/osdep.h
> index 1cdc7e2..9f0da46 100644
> --- a/osdep.h
> +++ b/osdep.h
> @@ -90,6 +90,10 @@ void *qemu_memalign(size_t alignment, size_t size);
> void *qemu_vmalloc(size_t size);
> void qemu_vfree(void *ptr);
> 
> +#define QEMU_MADV_DONTNEED 1

(1 << 0)

There are probably more to follow. I'm also not sure if it wouldn't make sense 
to just directly map qemu_madvise and real madvise bits. Something like

#ifdef MADV_DONTNEED
#define QEMU_MADV_DONTNEED   (1 << 0)
#else
#define QEMU_MADV_DONTNEED   0
#endif

Unless of course a flag is mandatory - but I don't think any of the madvise 
bits are.

> +
> +int qemu_madvise(void *addr, size_t len, int advice);
> +
> int qemu_create_pidfile(const char *filename);
> 
> #ifdef _WIN32
> diff --git a/vl.c b/vl.c
> index 3f45aa9..d352d18 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -80,9 +80,6 @@
> #include <net/if.h>
> #include <syslog.h>
> #include <stropts.h>
> -/* See MySQL bug #7156 (http://bugs.mysql.com/bug.php?id=7156) for
> -   discussion about Solaris header problems */
> -extern int madvise(caddr_t, size_t, int);

Does Solaris have posix_madvise? Otherwise it would still require this header 
hack, no?


Alex