Re: [PATCH v3 7/8] KVM:X86: Add XSS bit 11 and 12 support for CET xsaves/xrstors.

[Yang Weijiang]

On Thu, Feb 28, 2019 at 08:25:12AM -0800, Sean Christopherson wrote:
> On Mon, Feb 25, 2019 at 09:27:15PM +0800, Yang Weijiang wrote:
> > For Guest XSS, right now, only bit 11(user states) and bit 12
> > (supervisor states) are supported, if other bits are being set,
> > need to modify KVM_SUPPORTED_XSS macro to have support.
> 
> The changelog should describe what the change does directly.  Referencing
> specific bits implies that the code is explicitly checking for said bits,
> which it does not.  And there's no need to advise readers on how to add
> more bits in the future, e.g. the KVM_SUPPORTED_XSS macro may not exist
> the next time new bits are added.  Just cover what the patch does and why.
> 
> Something like:
> 
> KVM: x86: Allow the guest to set supported bits in XSS
> 
> ...now that KVM supports setting CET related bits.  Previously, KVM did
> not support setting any bits in XSS and so hardcoded its check to inject
> a #GP if the guest attempted to write a non-zero value to IA32_XSS.
>
good point! Will change these description.
> > 
> > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/vmx.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index d32cee9ee079..68908ed7b151 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -47,6 +47,7 @@
> >  #include <asm/virtext.h>
> >  #include <asm/mce.h>
> >  #include <asm/fpu/internal.h>
> > +#include <asm/fpu/types.h>
> >  #include <asm/perf_event.h>
> >  #include <asm/debugreg.h>
> >  #include <asm/kexec.h>
> > @@ -4336,12 +4337,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  	case MSR_IA32_XSS:
> >  		if (!vmx_xsaves_supported())
> >  			return 1;
> > +
> >  		/*
> > -		 * The only supported bit as of Skylake is bit 8, but
> > -		 * it is not supported on KVM.
> > +		 * Check bits being set are supported in KVM.
> 
> I'd drop the comment altogether, it's pretty obvious from the code that
> were checking which bits are supported.
you won't see these redundancies in next version ;)
> >  		 */
> > -		if (data != 0)
> > +		if (data & ~kvm_supported_xss())
> >  			return 1;
> > +
> >  		vcpu->arch.ia32_xss = data;
> >  		if (vcpu->arch.ia32_xss != host_xss)
> >  			add_atomic_switch_msr(vmx, MSR_IA32_XSS,
> > -- 
> > 2.17.1
> >