Re: [Qemu-devel] [PATCH v2 2/6] x86: Mask mtrr mask based on CPU physical address limits

[Michael S. Tsirkin]

On Mon, Jul 04, 2016 at 08:16:05PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> The CPU GPs if we try and set a bit in a variable MTRR mask above
> the limit of physical address bits on the host.  We hit this
> when loading a migration from a host with a larger physical
> address limit than our destination (e.g. a Xeon->i7 of same
> generation) but previously used to get away with it
> until 48e1a45 started checking that msr writes actually worked.
> 
> It seems in our case the GP probably comes from KVM emulating
> that GP.
> 
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>

Why don't we mask with host bits? This is the kvm limitation,
isn't it? This will make it possible to CC stable as well.

> ---
>  target-i386/kvm.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index f3698f1..6429205 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -1677,6 +1677,8 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
>              }
>          }
>          if (has_msr_mtrr) {
> +            uint64_t phys_mask = MAKE_64BIT_MASK(0, cpu->phys_bits);
> +
>              kvm_msr_entry_add(cpu, MSR_MTRRdefType, env->mtrr_deftype);
>              kvm_msr_entry_add(cpu, MSR_MTRRfix64K_00000, env->mtrr_fixed[0]);
>              kvm_msr_entry_add(cpu, MSR_MTRRfix16K_80000, env->mtrr_fixed[1]);
> @@ -1690,10 +1692,15 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
>              kvm_msr_entry_add(cpu, MSR_MTRRfix4K_F0000, env->mtrr_fixed[9]);
>              kvm_msr_entry_add(cpu, MSR_MTRRfix4K_F8000, env->mtrr_fixed[10]);
>              for (i = 0; i < MSR_MTRRcap_VCNT; i++) {
> +                /* The CPU GPs if we write to a bit above the physical limit 
> of
> +                 * the host CPU (and KVM emulates that)
> +                 */
> +                uint64_t mask = env->mtrr_var[i].mask;
> +                mask &= phys_mask;
> +
>                  kvm_msr_entry_add(cpu, MSR_MTRRphysBase(i),
>                                    env->mtrr_var[i].base);
> -                kvm_msr_entry_add(cpu, MSR_MTRRphysMask(i),
> -                                  env->mtrr_var[i].mask);
> +                kvm_msr_entry_add(cpu, MSR_MTRRphysMask(i), mask);
>              }
>          }
>  
> -- 
> 2.7.4