Re: [Qemu-devel] [PATCH 10/10] linux-user: init_guest_space: Try to make ARM space+commpage continuous

[Luke Shumaker <lukeshu@lukeshu.com>]

On Fri, 02 Mar 2018 09:13:12 -0500,
Peter Maydell wrote:
> On 28 December 2017 at 18:08, Luke Shumaker <lukeshu@lukeshu.com> wrote:
> > +        guest_full_size =
> > +            (0xffff0f00 & qemu_host_page_mask) + qemu_host_page_size;
                        ^
> I think this is probably more clearly written as 0x100000000ULL,
> since rounding down to the host-page-size then adding the host-page-size
> gets us the full 32-bit size of the guest address space.

Wait, is that right?  Isn't that only true if qemu_host_page_size is
at least 8KiB (16 bits), enough to fill the zero in the middle?  Won't
a typical qemu_host_page_size be only 4KiB?

> That shows up that there's a potential problem here if the host
> is 32-bit, because in that case guest_full_size (being only unsigned
> long) will be 0, and we'll end up trying an mmap with an incorrect size.
> 
> > +        host_full_size = guest_full_size - guest_start;
> > +        real_start = (unsigned long)
> > +            mmap(NULL, host_full_size, PROT_NONE, flags, -1, 0);
> 
> I think the general approach is right, though. Sorry it took so long
> for us to get to reviewing this patchset.

It's all good.  I'm amazed at the amount of traffic qemu-devel gets!

> Incidentally, this code would be rather less complicated if it didn't
> have to account for qemu_host_page_size not actually being the host
> page size (since then you couldn't get a return from mmap() that wasn't
> aligned properly). Does anybody know why we allow the user to specify
> it on the command line? (git revision history doesn't help, it just says
> there's been a -pagesize argument since commit 54936004fddc5 in 2003,
> right back when mmap emulation was first added...)

I have no idea, I just assumed that it was a feature useful to people
far smarter than me.

-- 
Happy hacking,
~ Luke Shumaker