linux-kernel - Re: [PATCH v2 7/7] KVM:X86: Enable MSR_IA32_XSS bit 11 and 12 for CET xsaves/xrstors.

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Date:   Fri, 25 Jan 2019 15:03:34 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, jmattson@...gle.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mst@...hat.com,
        yu-cheng.yu@...el.com, yi.z.zhang@...el.com, hjl.tools@...il.com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v2 7/7] KVM:X86: Enable MSR_IA32_XSS bit 11 and 12 for
 CET xsaves/xrstors.

On Wed, Jan 23, 2019 at 04:59:09AM +0800, Yang Weijiang wrote:
> For kvm Guest OS, right now, only bit 11(user mode CET) and bit 12
> (supervisor CET) are supported in XSS MSR, if other bits are being set,
> the write to XSS will be skipped.
> 
> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/vmx.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9c8cecac80ea..25ac22b3923a 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>
> @@ -4334,12 +4335,16 @@ 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.
> +		 * Right now, only support XSS_CET_U[bit 11] and
> +		 * XSS_CET_S[bit 12] in MSR_IA32_XSS.
>  		 */
> -		if (data != 0)
> +
> +		if (!vmx_guest_cet_cap(vcpu) ||

This isn't super intuitive, as evidenced by the additional comment.
If you mask off the unsupported bits then you can make a clean check and
don't need a comment, e.g.:

		supported = kvm_supported_xss();
		if (!guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) &&
		    !guest_cpuid_has(vcpu, X86_FEATURE_IBT))
			supported &= ~(XSS_CET_S | XSS_CET_U);

		if (data & ~supported)
			return 1;

> +		    data & ~(KVM_SUPPORTED_XSS & host_xss))

Didn't you add kvm_supported_xss() in an earlier patch?  Might as well
use it here.

>  			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
>