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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel