Re: [PATCH v7 04/26] x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL_DYNAMIC xfeature set

[Maxim Levitsky]

On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> Define new XFEATURE_MASK_KERNEL_DYNAMIC set including the features can be

I am not sure though that this name is correct, but I don't know if I can
suggest a better name.

> optionally enabled by kernel components, i.e., the features are required by
> specific kernel components. 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 xsaves/xrstors operating xfeature set
> (XCR0 | XSS), but the relevant CPU feature, i.e., supervisor shadow stack, is
> not enabled in host kernel so 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 */

I suggest to explain this a bit better. How about something like that:

"Kernel features that are not enabled by default for all processes, but can
be still used by some processes, for example to support guest virtualization"

But feel free to keep it as is or propose something else. IMHO this will
be confusing this way or another.


Another question: kernel already has a notion of 'independent features'
which are currently kernel features that are enabled in IA32_XSS but not present in 'fpu_kernel_cfg.max_features'

Currently only 'XFEATURE_LBR' is in this set. These features are saved/restored manually
from independent buffer (in case of LBRs, perf code cares for this).

Does it make sense to add CET_S to there as well instead of having XFEATURE_MASK_KERNEL_DYNAMIC, and maybe rename the
'XFEATURE_MASK_INDEPENDENT' to something like 'XFEATURES_THE_KERNEL_DOESNT_CARE_ABOUT'
(terrible name, but you might think of a better name)


> +#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 b57d909facca..ba4172172afd 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;



Best regards,
	Maxim Levitsky