Re: [Qemu-devel] [PATCH] KVM: x86: Add host physical address width capability

[Laszlo Ersek]

On 07/09/15 08:09, Paolo Bonzini wrote:
> 
> 
> On 09/07/2015 00:36, Bandan Das wrote:
>> Let userspace inquire the maximum physical address width
>> of the host processors; this can be used to identify maximum
>> memory that can be assigned to the guest.
>>
>> Reported-by: Laszlo Ersek <address@hidden>
>> Signed-off-by: Bandan Das <address@hidden>
>> ---
>>  arch/x86/kvm/x86.c       | 3 +++
>>  include/uapi/linux/kvm.h | 1 +
>>  2 files changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index bbaf44e..97d6746 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2683,6 +2683,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long 
>> ext)
>>      case KVM_CAP_NR_MEMSLOTS:
>>              r = KVM_USER_MEM_SLOTS;
>>              break;
>> +    case KVM_CAP_PHY_ADDR_WIDTH:
>> +            r = boot_cpu_data.x86_phys_bits;
>> +            break;
> 
> Userspace can just use CPUID, can't it?

I believe KVM's cooperation is necessary, for the following reason:

The truncation only occurs when the guest-phys <-> host-phys translation
is done in hardware, *and* the phys bits of the host processor are
insufficient to represent the highest guest-phys address that the guest
will ever face.

The first condition (of course) means that the truncation depends on EPT
being enabled. (I didn't test on AMD so I don't know if RVI has the same
issue.) If EPT is disabled, either because the host processor lacks it,
or because the respective kvm_intel module parameter is set so, then the
issue cannot be experienced.

Therefore I believe a KVM patch is necessary.

However, this specific patch doesn't seem sufficient; it should also
consider whether EPT is enabled. (And the ioctl should be perhaps
renamed to reflect that -- what QEMU needs to know is not the raw
physical address width of the host processor, but whether that width
will cause EPT to silently truncate high guest-phys addresses.)

Thanks
Laszlo

> 
> Paolo
> 
>>      case KVM_CAP_PV_MMU:    /* obsolete */
>>              r = 0;
>>              break;
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 716ad4a..e7949a1 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -817,6 +817,7 @@ struct kvm_ppc_smmu_info {
>>  #define KVM_CAP_DISABLE_QUIRKS 116
>>  #define KVM_CAP_X86_SMM 117
>>  #define KVM_CAP_MULTI_ADDRESS_SPACE 118
>> +#define KVM_CAP_PHY_ADDR_WIDTH 119
>>  
>>  #ifdef KVM_CAP_IRQ_ROUTING
>>  
>>