Re: [Qemu-devel] [PATCH v3 4/4] x86: Set physical address bits based on host

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Tue, Jul 05, 2016 at 08:03:18PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > A special case based on the previous phys-bits property; if it's
> > the magic value 0 then use the hosts capabilities.
> > 
> > We can also use the value we read from the host to check the users
> > explicitly set value and warn them if it doesn't match.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  target-i386/cpu.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 53 insertions(+)
> > 
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index f33cf58..6ebd26b 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -2952,7 +2952,60 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
> >             & CPUID_EXT2_AMD_ALIASES);
> >      }
> >  
> > +    /* For 64bit systems think about the number of physical bits to present.
> > +     * ideally this should be the same as the host; anything other than matching
> > +     * the host can cause incorrect guest behaviour.
> > +     * QEMU used to pick the magic value of 40 bits that corresponds to
> > +     * consumer AMD devices but nothing esle.
> > +     */
> >      if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
> > +        if (kvm_enabled()) {
> > +            /* Read the hosts physical address size, and compare it to what we
> > +             * were asked for; note old machine types default to 40 bits
> > +             */
> 
> Isn't the "old machine types default to 40 bits" part obsolete,
> now that you use 9999 to indicate it was not set explicitly?

Yes it is; gone.

> Also, the observation makes me confused: I know the old machine
> types default to 40 bits, but I don't know why I need to know
> that to understand the host_cpuid() logic below.

Yes that comment also made more sense the way it used to be arranged.

> 
> > +            uint32_t eax;
> > +            uint32_t host_phys_bits = 0;
> > +            static bool warned;
> > +
> > +            host_cpuid(0x80000000, 0, &eax, NULL, NULL, NULL);
> > +            if (eax >= 0x80000008) {
> > +                host_cpuid(0x80000008, 0, &eax, NULL, NULL, NULL);
> > +                /* Note: According to AMD doc 25481 rev 2.34 they have a field
> > +                 * at 23:16 that can specify a maximum physical address bits for
> > +                 * the guest that can override this value; but I've not seen
> > +                 * anything with that set.
> > +                 */
> > +                host_phys_bits = eax & 0xff;
> > +            } else {
> > +                /* It's an odd 64 bit machine that doesn't have the leaf for
> > +                 * physical address bits; fall back to 36 that's most older
> > +                 * Intel.
> > +                 */
> > +                host_phys_bits = 36;
> > +            }
> 
> I would love to see this logic moved inside a
> x86_host_phys_bits() function.

Done.

> > +
> > +            if (cpu->phys_bits == 0) {
> > +                /* The user asked for us to use the host physical bits */
> > +                cpu->phys_bits = host_phys_bits;
> > +            }
> > +
> > +            /* Print a warning if the user set it to a value that's not the
> > +             * host value.
> > +             */
> > +            if (cpu->phys_bits != host_phys_bits && cpu->phys_bits != 9999 &&
> > +                !warned) {
> > +                error_report("Warning: Host physical bits (%u)"
> > +                                 " does not match phys_bits (%u)",
> > +                                 host_phys_bits, cpu->phys_bits);
> > +                warned = true;
> 
> The name of the user-visible property is "phys-bits", not
> phys_bits. Maybe we could say "does not match phys-bits
> property".

Done.

Dave

> 
> > +            }
> > +        } else {
> > +            if (cpu->phys_bits == 0) {
> > +                error_setg(errp, "phys_bits can not be read from the host in"
> > +                                 " TCG mode");
> > +                return;
> > +            }
> > +        }
> >          /* 9999 is a special meaning 'use the old default',
> >           */
> >          if (cpu->phys_bits == 9999) {
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK