Re: [Qemu-devel] [PATCH v4 5/5] x86: Set physical address bits based on host

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

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Fri, Jul 08, 2016 at 04:01:39PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Add the host-phys-bits boolean property, if true, take phys-bits
> > from the hosts physical bits value, overriding either the default
> > or the user specified value.
> > 
> > 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>
> > ---
> [...]
> > @@ -2952,28 +2977,57 @@ 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 else.
> > +     */
> > +    if (cpu->host_phys_bits && !kvm_enabled()) {
> > +        error_setg(errp, "phys-bits can not be read from the host in"
> > +                         " TCG mode");
> > +        return;
> > +    }
> > +
> >      if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
> > -        /* 0 is a special meaning 'use the old default', which matches
> > -         * the value used by TCG (40).
> > -         */
> > -        if (cpu->phys_bits == 0) {
> > -            cpu->phys_bits = TCG_PHYS_ADDR_BITS;
> > -        }
> >          if (kvm_enabled()) {
> > -            if (cpu->phys_bits > TARGET_PHYS_ADDR_SPACE_BITS ||
> > -                cpu->phys_bits < 32) {
> > +            uint32_t host_phys_bits = x86_host_phys_bits();
> > +            static bool warned;
> > +
> > +            if (cpu->host_phys_bits) {
> > +                /* 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 != 0 &&
> > +                !warned) {
> > +                error_report("Warning: Host physical bits (%u)"
> > +                                 " does not match phys-bits property (%u)",
> > +                                 host_phys_bits, cpu->phys_bits);
> > +                warned = true;
> > +            }
> > +
> > +            if (cpu->phys_bits &&
> > +                (cpu->phys_bits > TARGET_PHYS_ADDR_SPACE_BITS ||
> > +                cpu->phys_bits < 32)) {
> >                  error_setg(errp, "phys-bits should be between 32 and %u "
> >                                   " (but is %u)",
> >                                   TARGET_PHYS_ADDR_SPACE_BITS, cpu->phys_bits);
> >                  return;
> >              }
> >          } else {
> > -            if (cpu->phys_bits != TCG_PHYS_ADDR_BITS) {
> > +            if (cpu->phys_bits && cpu->phys_bits != TCG_PHYS_ADDR_BITS) {
> >                  error_setg(errp, "TCG only supports phys-bits=%u",
> >                                    TCG_PHYS_ADDR_BITS);
> >                  return;
> >              }
> >          }
> > +        if (cpu->phys_bits == 0 && !cpu->host_phys_bits) {
> 
> Why the !cpu->host_phys_bits check? It seems to be impossible to
> have (cpu->host_phys_bits == true && cpu->phys_bits == 0) here.
> 
> > +            cpu->phys_bits = TCG_PHYS_ADDR_BITS;
> > +        }
> >      } else {
> >          /* For 32 bit systems don't use the user set value, but keep
> >           * phys_bits consistent with what we tell the guest.
> 
> Shouldn't we return error if host-phys-bits is set in 32-bit
> mode?

I've just realised there's a reason that erroring in this case is a problem.
Imagine a future (or downstream) machine type that made host-phys-bits the default;
how would it run with a 32bit CPU?

Dave

> > @@ -3290,6 +3344,7 @@ static Property x86_cpu_properties[] = {
> >      DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
> >      DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
> >      DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),
> > +    DEFINE_PROP_BOOL("host-phys-bits", X86CPU, host_phys_bits, false),
> >      DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),
> >      DEFINE_PROP_UINT32("level", X86CPU, env.cpuid_level, 0),
> >      DEFINE_PROP_UINT32("xlevel", X86CPU, env.cpuid_xlevel, 0),
> > diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> > index 9d79146..3c4e64a 100644
> > --- a/target-i386/cpu.h
> > +++ b/target-i386/cpu.h
> > @@ -1184,6 +1184,9 @@ struct X86CPU {
> >      /* if true fill the top bits of the MTRR_PHYSMASKn variable range */
> >      bool fill_mtrr_mask;
> >  
> > +    /* if true override the phys_bits value with a value read from the host */
> > +    bool host_phys_bits;
> > +
> >      /* Number of physical address bits supported */
> >      uint32_t phys_bits;
> >  
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK