Re: [Xen-devel] [PATCH v3 1/2] x86/hvm: introduce cr{0, 4}_host_mask to store trapped bits of CR accesses

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/21/2018 03:27 AM, Roger Pau Monné wrote:
> On Tue, Feb 20, 2018 at 07:23:44PM -0500, Boris Ostrovsky wrote:
>> On 02/20/2018 03:56 AM, Roger Pau Monne wrote:
>>> At the moment this is currently set at VMC{S/B} creation and not changed,
>>> but further patches are going to change the CR4 mask at runtime.
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>>> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>>> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
>>> ---
>>> Changes since v2:
>>>  - Introduce masks only for CR4 and CR0.
>>>
>>> Changes since v1:
>>>  - New in this version.
>>> ---
>>>  xen/arch/x86/hvm/svm/vmcb.c    | 1 +
>>>  xen/arch/x86/hvm/vmx/vmcs.c    | 1 +
>>>  xen/include/asm-x86/hvm/vcpu.h | 4 ++++
>>>  3 files changed, 6 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/hvm/svm/vmcb.c b/xen/arch/x86/hvm/svm/vmcb.c
>>> index 0e6cba5b7b..beeafad235 100644
>>> --- a/xen/arch/x86/hvm/svm/vmcb.c
>>> +++ b/xen/arch/x86/hvm/svm/vmcb.c
>>> @@ -169,6 +169,7 @@ static int construct_vmcb(struct vcpu *v)
>>>      vmcb->tr.base = 0;
>>>      vmcb->tr.limit = 0xff;
>>>  
>>> +    v->arch.hvm_vcpu.cr0_host_mask = v->arch.hvm_vcpu.cr4_host_mask = ~0UL;
>>>      v->arch.hvm_vcpu.guest_cr[0] = X86_CR0_PE | X86_CR0_ET;
>>>      hvm_update_guest_cr(v, 0);
>>
>> Is there a reason for setting those fields? SVM won't use them, will it?
> No, SVM won't use them but IMHO it's good for coherency to also set
> them. The initial plan was to do something similar for SVM, but I
> couldn't find how to do selective CR4 trapping for SVM.
>
> I could move this inside of arch_vmx_struct if it seems better, but I
> was expecting to be able to do something similar for SVM in order to
> avoid needlessly trapping CR4.

I don't believe SVM has a similar feature. The closest it comes to this
is clean bits, but that's still rather different.

So it seems to me you are targeting something very much VMX-specific and
so I think it should indeed be part of arch_vmx_struct.

-boris


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