Re: [PATCH v8 04/26] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL_DYNAMIC xfeature set

[Maxim Levitsky]

On Thu, 2023-12-21 at 09:02 -0500, Yang Weijiang wrote:
> Define a new XFEATURE_MASK_KERNEL_DYNAMIC mask to specify the features
> that can be optionally enabled by kernel components. This is similar to
> XFEATURE_MASK_USER_DYNAMIC in that it contains optional xfeatures that
> can allows the FPU buffer to be dynamically sized. The difference is that
> the KERNEL variant contains supervisor features and will be enabled by
> kernel components that need them, and not directly by the user. Currently
> it's used by KVM to configure guest dedicated fpstate for calculating
> the xfeature and fpstate storage size etc.
> 
> The kernel dynamic xfeatures now only contain XFEATURE_CET_KERNEL, which
> is supported by host as they're enabled in kernel XSS MSR setting but
> relevant CPU feature, i.e., supervisor shadow stack, is not enabled in
> host kernel therefore it can be omitted for normal fpstate by default.
> 
> Remove the kernel dynamic feature from fpu_kernel_cfg.default_features
> so that the bits in xstate_bv and xcomp_bv are cleared and xsaves/xrstors
> can be optimized by HW for normal fpstate.
> 
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/include/asm/fpu/xstate.h | 5 ++++-
>  arch/x86/kernel/fpu/xstate.c      | 1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> index 3b4a038d3c57..a212d3851429 100644
> --- a/arch/x86/include/asm/fpu/xstate.h
> +++ b/arch/x86/include/asm/fpu/xstate.h
> @@ -46,9 +46,12 @@
>  #define XFEATURE_MASK_USER_RESTORE	\
>  	(XFEATURE_MASK_USER_SUPPORTED & ~XFEATURE_MASK_PKRU)
>  
> -/* Features which are dynamically enabled for a process on request */
> +/* Features which are dynamically enabled per userspace request */
>  #define XFEATURE_MASK_USER_DYNAMIC	XFEATURE_MASK_XTILE_DATA
>  
> +/* Features which are dynamically enabled per kernel side request */
> +#define XFEATURE_MASK_KERNEL_DYNAMIC	XFEATURE_MASK_CET_KERNEL
> +
>  /* All currently supported supervisor features */
>  #define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \
>  					    XFEATURE_MASK_CET_USER | \
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 03e166a87d61..ca4b83c142eb 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -824,6 +824,7 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
>  	/* Clean out dynamic features from default */
>  	fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features;
>  	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
> +	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_KERNEL_DYNAMIC;
>  
>  	fpu_user_cfg.default_features = fpu_user_cfg.max_features;
>  	fpu_user_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;


I still think that we should consider adding XFEATURE_MASK_CET_KERNEL to
XFEATURE_MASK_INDEPENDENT or at least have a good conversation on why this doesn't make sense,
but I also don't intend to fight over this, as long as the code works.

Best regards,
	Maxim Levitsky