Re: [Qemu-devel] [PATCH] allow more than 1T in KVM x86 guest

[Anthony Liguori]

On 09/15/2010 12:13 PM, Andrea Arcangeli wrote:
> Subject: allow more than 1T in KVM x86 guest
>
> From: Andrea Arcangeli<address@hidden>
>
> When host supports 48 bits of physical address reflect that in the guest cpuid
> to allow the guest to use more than 1TB of RAM.
>
> The migration code should probably be updated accordingly checking if the size
> of the guest ram is bigger than the migration target cpuid 0x80000008 limit and
> failing migration in that case. (not a real practical issue, I don't
> see many people migrating>1T guests yet :)
>
> The comment below refers to a 42 bit limit on exec.c, but I didn't identify
> what the comment refers to yet. At least now guest should be able to use 4TB.
>    

target-i386/cpu.h

#ifdef TARGET_X86_64
#define TARGET_PHYS_ADDR_SPACE_BITS 52
/* ??? This is really 48 bits, sign-extended, but the only thing
   accessible to userland with bit 48 set is the VSYSCALL, and that
   is handled via other mechanisms.  */
#define TARGET_VIRT_ADDR_SPACE_BITS 47
#else
#define TARGET_PHYS_ADDR_SPACE_BITS 36
#define TARGET_VIRT_ADDR_SPACE_BITS 32
#endif

The macros are then used in exec.c

Regards,

Anthony Liguori

> Signed-off-by: Andrea Arcangeli<address@hidden>
> ---
>
> diff --git a/target-i386/cpuid.c b/target-i386/cpuid.c
> index d63fdcb..462e709 100644
> --- a/target-i386/cpuid.c
> +++ b/target-i386/cpuid.c
> @@ -1189,6 +1189,12 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>               /* 64 bit processor */
>   /* XXX: The physical address space is limited to 42 bits in exec.c. */
>               *eax = 0x00003028;        /* 48 bits virtual, 40 bits physical */
> +            if (kvm_enabled()) {
> +                uint32_t _eax;
> +                host_cpuid(0x80000000, 0,&_eax, NULL, NULL, NULL);
> +                if (_eax>= 0x80000008)
> +                    host_cpuid(0x80000008, 0, eax, NULL, NULL, NULL);
> +            }
>           } else {
>               if (env->cpuid_features&  CPUID_PSE36)
>                   *eax = 0x00000024; /* 36 bits physical */
>
>