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

[Eduardo Habkost <ehabkost@redhat.com>]

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?

> @@ -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