Re: [PATCH RFC 7/7] kvm: x86: AMX XCR0 support for guest

[Sean Christopherson <seanjc@google.com>]

On Sun, Feb 07, 2021, Jing Liu wrote:
> Two XCR0 bits are defined for AMX to support XSAVE mechanism.
> Bit 17 is for tilecfg and bit 18 is for tiledata.

This fails to explain why they must be set in tandem.  Out of curisoity, assuming
they do indeed need to be set/cleared as a pair, what's the point of having two
separate bits?

> Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
> ---
>  arch/x86/kvm/x86.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bfbde877221e..f1c5893dee18 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -189,7 +189,7 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
>  #define KVM_SUPPORTED_XCR0     (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
>  				| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
>  				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> -				| XFEATURE_MASK_PKRU)
> +				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>  
>  u64 __read_mostly host_efer;
>  EXPORT_SYMBOL_GPL(host_efer);
> @@ -946,6 +946,12 @@ static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
>  		if ((xcr0 & XFEATURE_MASK_AVX512) != XFEATURE_MASK_AVX512)
>  			return 1;
>  	}
> +
> +	if (xcr0 & XFEATURE_MASK_XTILE) {
> +		if ((xcr0 & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE)
> +			return 1;
> +	}
> +
>  	vcpu->arch.xcr0 = xcr0;
>  
>  	if ((xcr0 ^ old_xcr0) & XFEATURE_MASK_EXTEND)
> -- 
> 2.18.4
>