Re: [PATCH] kvm tools: mmap guest kernel instead of reading it into memory

[Avi Kivity <avi@redhat.com>]

On 12/12/2011 08:14 PM, Sasha Levin wrote:
> On Mon, 2011-12-12 at 17:59 +0200, Pekka Enberg wrote:
> > On Mon, Dec 12, 2011 at 4:47 PM, Sasha Levin <levinsasha928@gmail.com> wrote:
> > > +       /* mmap the actual kernel */
> > > +       kvm->bz_fd = dup(fd_kernel);
> > > +       kvm->bz_len = st.st_size;
> > > +       setup_end = ALIGN(setup_size - PAGE_SIZE, PAGE_SIZE); /* Need it aligned to PAGE_SIZE */
> > > +       kvm->bz_start = mmap(NULL, kvm->bz_len, PROT_READ | PROT_WRITE,
> > > +                               MAP_PRIVATE, kvm->bz_fd, setup_end);
> > >
> > > -       while ((nr = read(fd_kernel, p, 65536)) > 0)
> > > -               p += nr;
> > > +       /* NOP everything before the kernel start */
> > > +       memset(kvm->bz_start, 0x90, setup_size - setup_end);
> > 
> > So what's the deal with this NOP thing? It really needs a comment that
> > explains it all.
>
> Right, I'll explain it here and if it sounds right to you I'll add it
> into the patch.
>
> Since the start of the actual kernel image is somewhere into the
> bzImage, and is not aligned to anything, we can't mmap() directly to the
> beginning of it.
>
> So what we do is mmap the kernel with <PAGE_SIZE bytes before it which
> belong to the setup code.
>
> KVM expects page aligned pointers for both in-guest physical memory
> start, and the corresponding userspace address. This means that we can't
> simply pass an offset within the memory we mapped before since it won't
> be page aligned.
>
> The solution is to NOP the bytes which belong to the setup code right
> before the kernel starts. In practice it means <PAGE_SIZE NOPs before
> actual kernel code starts running.

Can't you just adjust rip to point to the starting code?

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