Re: [Xen-devel] Re: [PATCH]: Allow tools to map arbitrarily large machphys_mfn_list on 32bit dom0

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Mon, 2011-03-14 at 15:55 +0000, Jan Beulich wrote:
> >>> On 14.03.11 at 16:19, Gianni Tedesco <gianni.tedesco@xxxxxxxxxx> wrote:
> > 
> > This permits suspend/resume to work with 32bit dom0/tools. AFAICT the
> > limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since that refers to a
> > limit in 32bit guest compat mappings under 64bit hypervisors, not
> > userspace where there may be gigabytes of useful virtual space available
> > for this.
> > 
> > Suggested-by: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
> > Signed-off-by: Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>
> > 
> > diff -r 8b5cbccbc654 xen/arch/x86/x86_64/compat/mm.c
> > --- a/xen/arch/x86/x86_64/compat/mm.c       Mon Mar 14 14:59:27 2011 +0000
> > +++ b/xen/arch/x86/x86_64/compat/mm.c       Mon Mar 14 15:17:59 2011 +0000
> > @@ -161,9 +161,7 @@ int compat_arch_memory_op(int op, XEN_GU
> >          if ( copy_from_guest(&xmml, arg, 1) )
> >              return -EFAULT;
> >  
> > -        limit = (unsigned long)(compat_machine_to_phys_mapping +
> > -            min_t(unsigned long, max_page,
> > -                  MACH2PHYS_COMPAT_NR_ENTRIES(current->domain)));
> > +        limit = (unsigned long)(compat_machine_to_phys_mapping + max_page);
> 
> While doing this shouldn't hurt (except slightly for performance of
> the hypercall), I don't see why it's useful: For slots past
> MACH2PHYS_COMPAT_NR_ENTRIES(current->domain) you
> wouldn't read non-null page table entries anyway (up to
> RDWR_COMPAT_MPT_VIRT_END), so I don't see why the tools
> couldn't equally well do with what we have currently (after all
> they get told how many slots were filled).

In order to be able to migrate any guest the tools in domain 0 need to
see the entire of host M2P, not just the subset which the kernel sees
mapped into its hypervisor hole (which is what
MACH2PHYS_COMPAT_NR_ENTRIES represents).

The hypercall reads from the global compat M2P mapping, not the guest
kernel mapping of it, so it should read valid entries all the way up to
RDWR_COMPAT_MPT_VIRT_END, AFAICT.

Ian.

> 
> Jan
> 
> >          if ( limit > RDWR_COMPAT_MPT_VIRT_END )
> >              limit = RDWR_COMPAT_MPT_VIRT_END;
> >          for ( i = 0, v = RDWR_COMPAT_MPT_VIRT_START, last_mfn = 0;
> 
> 



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