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

[Paolo Bonzini]

On 09/07/2015 08:43, Laszlo Ersek wrote:
> 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.)

Right; if you want to consider whether EPT is enabled (which is the
right thing to do, albeit it makes for a much bigger patch) a KVM patch
is necessary.  In that case you also need to patch the API documentation.

Paolo