Re: [PATCH] x86: mmap kernel for direct kernel boot

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 06, 2026 at 01:02:33PM -0500, John Snow wrote:
> On Wed, Oct 21, 2026 at 12:02 PM John Snow <jsnow@redhat.com> wrote:
> >
> > On Wed, Oct 21, 2026 at 2:31 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
> > >
> > > Reduce memory footprint by mapping the kernel into memory instead of
> > > reading into allocated memory in the common case (no -dtb $file on the
> > > qemu command line).
> > >
> > > Cc: John Snow <jsnow@redhat.com>
> > > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> >
> > Tested-by: John Snow <jsnow@redhat.com>
> >
> 
> Ping, is someone going to pick up this patch?
> 
> --js

ok

> > Using this patch, I shifted about 19MiB of resources from RssAnon to
> > RssFile. This theoretically allows sharing resources between multiple
> > QEMU instances in the trivial case using -kernel and -initrd. This
> > matches how we already load initrd.
> >
> > --js
> >
> > > ---
> > >  hw/i386/x86-common.c | 25 ++++++++++++++++++++-----
> > >  1 file changed, 20 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/hw/i386/x86-common.c b/hw/i386/x86-common.c
> > > index fa18bf1aeceb..25e141aa8537 100644
> > > --- a/hw/i386/x86-common.c
> > > +++ b/hw/i386/x86-common.c
> > > @@ -911,16 +911,31 @@ void x86_load_linux(X86MachineState *x86ms,
> > >      }
> > >
> > >      setup  = g_malloc(setup_size);
> > > -    kernel = g_malloc(kernel_size);
> > >      fseek(f, 0, SEEK_SET);
> > >      if (fread(setup, 1, setup_size, f) != setup_size) {
> > >          fprintf(stderr, "fread() failed\n");
> > >          exit(1);
> > >      }
> > > -    fseek(f, 0, SEEK_SET);
> > > -    if (fread(kernel, 1, kernel_size, f) != kernel_size) {
> > > -        fprintf(stderr, "fread() failed\n");
> > > -        exit(1);
> > > +    if (dtb_filename) {
> > > +        /* read kernel into allocated memory, so we can realloc for the dtb */
> > > +        kernel = g_malloc(kernel_size);
> > > +        fseek(f, 0, SEEK_SET);
> > > +        if (fread(kernel, 1, kernel_size, f) != kernel_size) {
> > > +            fprintf(stderr, "fread() failed\n");
> > > +            exit(1);
> > > +        }
> > > +    } else {
> > > +        /* mmap kernel */
> > > +        GMappedFile *mapped_file;
> > > +        GError *gerr = NULL;
> > > +
> > > +        mapped_file = g_mapped_file_new(kernel_filename, false, &gerr);
> > > +        if (!mapped_file) {
> > > +            fprintf(stderr, "qemu: error reading kernel %s: %s\n",
> > > +                    kernel_filename, gerr->message);
> > > +            exit(1);
> > > +        }
> > > +        kernel = (uint8_t *)g_mapped_file_get_contents(mapped_file);
> > >      }
> > >      fclose(f);
> > >
> > > --
> > > 2.56.0
> > >