Re: [Xen-devel] [PATCH 2/3] x86/HVM: support (emulate) UMIP

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/07/2016 06:29 AM, Jan Beulich wrote:
>>>> On 06.12.16 at 17:23, <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 12/06/2016 06:44 AM, Jan Beulich wrote:
>>> --- a/xen/arch/x86/cpuid.c
>>> +++ b/xen/arch/x86/cpuid.c
>>> @@ -154,6 +154,13 @@ static void __init calculate_hvm_feature
>>>      __set_bit(X86_FEATURE_APIC, hvm_featureset);
>>>  
>>>      /*
>>> +     * Xen can often provide UMIP emulation to HVM guests even if the host
>>> +     * doesn't have such functionality.
>>> +     */
>>> +    if ( cpu_has_vmx_dt_exiting || cpu_has_svm )
>>> +        __set_bit(X86_FEATURE_UMIP, hvm_featureset);
>> I don't think I understand how this is going to work for processors that
>> don't support UMIP.
>>
>> How, for example, can guest_cr[4] have X86_CR4_UMIP set on these
>> processors when CPUID will not show the feature being there?
> What we allow the guest to see and what we store into hardware
> registers are two different things: Note how svm_update_guest_cr()
> masks off X86_CR4_UMIP from the vale to be put into the VMCB.

So that was kind of my question --- why would a guest ever try to set
this bit? As far as it is concerned, UMIP is not available and the guest
is then trying to set an unsupported bit in cr4. And that should result
in a #GP.

-boris

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