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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 21, 2018 at 08:48:20AM -0500, Boris Ostrovsky wrote:
> 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.

Thanks, will change that and wait for further feedback before
resending.

Roger.

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