Re: [Qemu-devel] [PATCH v2 4/6] x86: Set physical address bits based on host

[Eduardo Habkost]

On Mon, Jul 04, 2016 at 08:16:07PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> A special case based on the previous phys-bits property; if it's
> the magic value 0 then use the hosts capabilities.
> 
> This becomes the default on new machine types.
> 
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
>  include/hw/i386/pc.h |  5 +++++
>  target-i386/cpu.c    | 36 +++++++++++++++++++++++++++++++++++-
>  2 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index d85e924..bf31609 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -379,6 +379,11 @@ bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t 
> *);
>          .driver = TYPE_X86_CPU,\
>          .property = "fill-mtrr-mask",\
>          .value = "off",\
> +    },\
> +    {\
> +        .driver = TYPE_X86_CPU,\
> +        .property = "phys-bits",\
> +        .value = "40",\
>      },
>  
>  #define PC_COMPAT_2_5 \
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 5737aba..d45d2a6 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -2957,6 +2957,40 @@ 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) {
> +        uint32_t eax;
> +        /* Read the hosts physical address size, and compare it to what we
> +         * were asked for; note old machine types default to 40 bits
> +         */
> +        uint32_t host_phys_bits = 0;
> +        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;
> +        }

Why do we need to calculate host_phys_bits when phys_bits is
already set? Shouldn't we put all the code above after the "if
(cpu->phys_bits)" check?

> +
> +        if (cpu->phys_bits == 0) {
> +            /* The user asked for us to use the host physical bits */
> +            cpu->phys_bits = host_phys_bits;
> +
> +        }
> +    }
>  
>      cpu_exec_init(cs, &error_abort);
>  
> @@ -3259,7 +3293,7 @@ static Property x86_cpu_properties[] = {
>      DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
>      DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
>      DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),
> -    DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 40),
> +    DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),

I would put this part (that sets the default to 0) and the
PC_COMPAT_2_6 part in a separate patch. This way we can include
the mechanism for setting phys-bits=0 even if we didn't reach a
conclusion about the proper pc-2.7 default yet.

>      DEFINE_PROP_UINT32("level", X86CPU, env.cpuid_level, 0),
>      DEFINE_PROP_UINT32("xlevel", X86CPU, env.cpuid_xlevel, 0),
>      DEFINE_PROP_UINT32("xlevel2", X86CPU, env.cpuid_xlevel2, 0),
> -- 
> 2.7.4
> 

-- 
Eduardo