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

[Sean Christopherson]

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.

> 
> 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.

>  		 */
> -		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
>