Re: [Qemu-devel] [PATCH] spapr: Correct RAM size calculation for HPT resizing

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Oct 10, 2017 at 05:21:53PM +0200, Laurent Vivier wrote:
> On 10/10/2017 16:44, Greg Kurz wrote:
> > On Wed, 11 Oct 2017 00:21:59 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> > 
> >> In order to prevent the guest from forcing the allocation of large amounts
> >> of qemu memory (or host kernel memory, in the case of KVM HV), we limit
> >> the size of Hashed Page Table (HPT) it is allowed to allocated, based on
> >> its RAM size.
> >>
> >> However, the current calculation is not correct: it only adds up the size
> >> of plugged memory, ignoring the base memory size.  This patch corrects it.
> >>
> >> While we're there, use get_plugged_memory_size() instead of directly
> >> calling pc_existing_dimms_capacity().  The only difference is that it
> >> will abort on failure, which is right: a failure here indicates something
> >> wrong within qemu.
> >>
> >> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> >> ---
> >>  hw/ppc/spapr_hcall.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> >> index 8d72bb7c1c..06af1b15c0 100644
> >> --- a/hw/ppc/spapr_hcall.c
> >> +++ b/hw/ppc/spapr_hcall.c
> >> @@ -494,7 +494,7 @@ static target_ulong h_resize_hpt_prepare(PowerPCCPU *cpu,
> >>          return H_PARAMETER;
> >>      }
> >>  
> >> -    current_ram_size = pc_existing_dimms_capacity(&error_fatal);
> >> +    current_ram_size = ram_size + get_plugged_memory_size();
> > 
> > current_ram_size is initialized earlier in this function:
> > 
> >     uint64_t current_ram_size = MACHINE(spapr)->ram_size;
> > 
> > which is is initialized to ram_size in vl.c. Why not doing:
> > 
> >     current_ram_size += get_plugged_memory_size();
> > 
> > ?
> 
> I agree, it seems like the original intend of the first patch...

Yes, I think so.  However, splitting the calculation like that
demonstrably misread someone reading the code (i.e. me), so I'm going
to just ditch the initializerin the new spin.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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