Re: [Qemu-devel] [PATCH v2 6/6] x86: Add sanity checks on phys_bits

[Dr. David Alan Gilbert]

* Eduardo Habkost (address@hidden) wrote:
> On Mon, Jul 04, 2016 at 08:16:09PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > Add some sanity checks on the phys-bits setting now that
> > the user can set it.
> >    a) That it's in a sane range (52..32)
> >    b) Warn if it mismatches the host and isn't the old default.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  target-i386/cpu.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index e15abea..5402002 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -2985,6 +2985,19 @@ static void x86_cpu_realizefn(DeviceState *dev, 
> > Error **errp)
> >              /* The user asked for us to use the host physical bits */
> >              cpu->phys_bits = host_phys_bits;
> >  
> > +        } else if (cpu->phys_bits > 52 || cpu->phys_bits < 32) {
> > +            error_setg(errp, "phys_bits should be between 32 and 52 or 0 
> > to"
> > +                             " use host size (but is %u)", cpu->phys_bits);
> > +            return;
> > +        }
> 
> This check belongs to patch 1/6, doesn't it?

Yes, I can move it to there.

> Here we have the same magic number (52), and I don't know where
> it came from. Maybe it should become a (documented) macro?
> 
> Also, won't this make the "phys_bits < 52" check added by patch
> 3/6 unnecessary?

Possibly; although it did feel safer to put that in where we were generating
the bitmask.

> > +        /* Print a warning if the user set it to a value that's not the
> > +         * host value; ignore the magic value 40 because it may well just
> > +         * be the old machine type.
> > +         */
> 
> With this, we won't print a warning if "phys-bits=40" is set
> explicitly. If we want to disable the warning only for the old
> machine-types, we can add a boolean flag that disables it.

Yes, I can do that.

> > +        if (cpu->phys_bits != host_phys_bits && cpu->phys_bits != 40) {
> > +            fprintf(stderr, "Warning: Host physical bits (%u)"
> > +                            " does not match phys_bits (%u)\n",
> > +                            host_phys_bits, cpu->phys_bits);
> 
> Shouldn't we use error_report() for this?
> 
> Also, this prints a warning for each VCPU. This is not the first
> time we want to print a warning only once (see
> x86_cpu_apic_id_from_index() in hw/i386/pc.c and ht_warned in
> target-i386/cpu.c). It looks like QEMU needs a warn_once()
> helper.

OK, will do.

Dave

> >          }
> >      } else {
> >          /* For 32 bit systems don't use the user set value, but keep
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Eduardo
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK