Re: [PATCH v3 3/8] KVM:CPUID: Add CPUID support for Guest CET

[Yang Weijiang]

On Mon, Mar 04, 2019 at 10:54:29AM -0800, Sean Christopherson wrote:
> On Mon, Mar 04, 2019 at 10:47:53AM -0800, Sean Christopherson wrote:
> > On Mon, Feb 25, 2019 at 09:27:11PM +0800, Yang Weijiang wrote:
> > > Guest CET SHSTK and IBT capability are reported via
> > > CPUID.(EAX=7, ECX=0):ECX[bit 7] and EDX[bit 20] respectively.
> > > Guest user mode and supervisor mode xsaves component size
> > > is reported via CPUID.(EAX=0xD, ECX=1):ECX[bit 11] and ECX[bit 12]
> > > respectively.
> > > 
> > > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/cpuid.c | 60 +++++++++++++++++++++++++++++++++-----------
> > >  arch/x86/kvm/x86.h   |  4 +++
> > >  2 files changed, 50 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index cb1aece25b17..5e05756cc6db 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -65,6 +65,16 @@ u64 kvm_supported_xcr0(void)
> > >  	return xcr0;
> > >  }
> > >  
> > > +u64 kvm_supported_xss(void)
> > > +{
> > > +	u64 xss;
> > > +
> > > +	rdmsrl(MSR_IA32_XSS, xss);
> > 
> > Honest question as I haven't thought through the flows: do we actually
> > need to restrict XSS based on what's enabled in the host?  This
> > conflicts with your other statements that CET features can be enabled
> > independent of host support.
> 
> Never mind, just saw Paolo's comment in a previous version about XSAVE
> being dependent on host XSS.  The below comment about caching still
> applies though.
>
In my WIP work, I use existing host_xss in vmx.c instead of reading MSR here to
incoperate host status.

> > And if we do incorporate the host status, the value should be read once
> > and cached unless we're expecting the host value to change dynamically,
> > e.g. see host_efer.
> > 
> > > +	xss &= KVM_SUPPORTED_XSS;
> > > +	return xss;
> > > +}
> > > +EXPORT_SYMBOL(kvm_supported_xss);
> > > +
> > >  #define F(x) bit(X86_FEATURE_##x)
> > >  
> > >  /* For scattered features from cpufeatures.h; we currently expose none */