Re: [Qemu-devel] [PATCH 2/7] QEMU does not currently support host pages that are larger than guest pages, likely due to glibc using fixed mmap requests.

[Richard Henderson]

On 06/19/2016 02:46 AM, Peter Maydell wrote:
> On 19 June 2016 at 01:11, Timothy Pearson
> <address@hidden> wrote:
> > Attempting to use host pages larger than the guest leads to
> > alignment errors during ELF load in the best case, and an
> > initialization failure inside NPTL in the worst case, causing
> > all fork() requests inside the guest to fail.
> >
> > Warn when thread space cannot be set up, and suggest reducing
> > host page size if applicable.
>
> This is supposed to work -- for instance the linux-user/mmap.c
> code has support for host pages and target pages not being the same.
> In particular for ARM guests TARGET_PAGE_SIZE is 1K but the
> host page size is 4K, so the config of "host page larger than
> guest" isn't untested.

You're right, it isn't untested, it's completely broken.

The arm 1k example isn't really realistic, because the actual arm kernel doles 
out 4k units, and that's what userspace was built to expect.  Try running i386 
on aarch64 (or ppc64) with a 64k page kernel.  Maps fail very very quickly.

This is one of the reasons why I think there's no way to do such emulation 
without softmmu for linux-user.


r~