> -----Original Message-----
> From: Gleb Natapov [mailto:[email protected]]
> Sent: Sunday, February 03, 2013 9:57 PM
> To: Xu, Dongxiao
> Cc: [email protected]
> Subject: Re: KVM: VMX: disable SMEP feature when guest is in non-paging
> mode
>
> On Fri, Feb 01, 2013 at 08:30:07AM -0000, Xu wrote:
> > SMEP is disabled if CPU is in non-paging mode in hardware.
> > However KVM always uses paging mode to emulate guest non-paging
> > mode with HAP.
> Not always, only if unrestricted mode is disabled, since vm86 mode, that
> is used otherwise, requires paging.
>
> > To emulate this behavior, SMEP needs to be manually
> > disabled when guest switches to non-paging mode.
> >
> > We met an issue that, SMP Linux guest with recent kernel (enable
> > SMEP support, for example, 3.5.3) would crash with triple fault if
> > setting unrestricted_guest=0. This is because KVM uses an identity
> > mapping page table to emulate the non-paging mode, where the page
> > table is set with USER flag. If SMEP is still enabled in this case,
> > guest will meet unhandlable page fault and then crash.
> >
> > Signed-off-by: Dongxiao Xu <[email protected]>
> > Signed-off-by: Xiantao Zhang <[email protected]>
> Reviewed-by: Gleb Natapov <[email protected]>
>
> But HAP is XEN terminology AFAIK. KVM speak is tdp (two dimensional
> paging). If would be nice to change it in the commit message and the
> comment before committing.
Thanks for reminding this. I've sent out the v2 patch to address this issue.
Thanks,
Dongxiao
>
> >
> > ---
> > arch/x86/kvm/vmx.c | 8 ++++++++
> > 1 files changed, 8 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 9120ae1..e82f20d 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -3155,6 +3155,14 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu,
> unsigned long cr4)
> > if (!is_paging(vcpu)) {
> > hw_cr4 &= ~X86_CR4_PAE;
> > hw_cr4 |= X86_CR4_PSE;
> > + /*
> > + * SMEP is disabled if CPU is in non-paging mode in
> > + * hardware. However KVM always uses paging mode to
> > + * emulate guest non-paging mode with HAP.
> > + * To emulate this behavior, SMEP needs to be manually
> > + * disabled when guest switches to non-paging mode.
> > + */
> > + hw_cr4 &= ~X86_CR4_SMEP;
> > } else if (!(cr4 & X86_CR4_PAE)) {
> > hw_cr4 &= ~X86_CR4_PAE;
> > }
>
> --
> Gleb.
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