Re: [PATCH v8 06/26] x86/fpu/xstate: Create guest fpstate with guest specific config

[Maxim Levitsky]

On Thu, 2023-12-21 at 09:02 -0500, Yang Weijiang wrote:
> Use fpu_guest_cfg to calculate guest fpstate settings, open code for
> __fpstate_reset() to avoid using kernel FPU config.
> 
> Below configuration steps are currently enforced to get guest fpstate:
> 1) Kernel sets up guest FPU settings in fpu__init_system_xstate().
> 2) User space sets vCPU thread group xstate permits via arch_prctl().
> 3) User space creates guest fpstate via __fpu_alloc_init_guest_fpstate()
>    for vcpu thread.
> 4) User space enables guest dynamic xfeatures and re-allocate guest
>    fpstate.
> 
> By adding kernel dynamic xfeatures in above #1 and #2, guest xstate area
> size is expanded to hold (fpu_kernel_cfg.default_features | kernel dynamic
> xfeatures | user dynamic xfeatures), then host xsaves/xrstors can operate
> for all guest xfeatures.
> 
> The user_* fields remain unchanged for compatibility with KVM uAPIs.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/kernel/fpu/core.c | 47 ++++++++++++++++++++++++++++++--------
>  1 file changed, 37 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 976f519721e2..0e0bf151418f 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -250,8 +250,6 @@ void fpu_reset_from_exception_fixup(void)
>  }
>  
>  #if IS_ENABLED(CONFIG_KVM)
> -static void __fpstate_reset(struct fpstate *fpstate, u64 xfd);
> -
>  static void fpu_init_guest_permissions(struct fpu_guest *gfpu)
>  {
>  	struct fpu_state_perm *fpuperm;
> @@ -272,25 +270,54 @@ static void fpu_init_guest_permissions(struct fpu_guest *gfpu)
>  	gfpu->perm = perm & ~FPU_GUEST_PERM_LOCKED;
>  }
>  
> -bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
> +static struct fpstate *__fpu_alloc_init_guest_fpstate(struct fpu_guest *gfpu)
>  {
> +	bool compacted = cpu_feature_enabled(X86_FEATURE_XCOMPACTED);
> +	unsigned int gfpstate_size, size;
>  	struct fpstate *fpstate;
> -	unsigned int size;
>  
> -	size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
> +	/*
> +	 * fpu_guest_cfg.default_size is initialized to hold all enabled
> +	 * xfeatures except the user dynamic xfeatures. If the user dynamic
> +	 * xfeatures are enabled, the guest fpstate will be re-allocated to
> +	 * hold all guest enabled xfeatures, so omit user dynamic xfeatures
> +	 * here.
> +	 */
> +	size = fpu_guest_cfg.default_size +
> +	       ALIGN(offsetof(struct fpstate, regs), 64);
> +
>  	fpstate = vzalloc(size);
>  	if (!fpstate)
> -		return false;
> +		return NULL;
> +	/*
> +	 * Initialize sizes and feature masks, use fpu_user_cfg.*
> +	 * for user_* settings for compatibility of exiting uAPIs.
> +	 */
> +	fpstate->size		= gfpstate_size;
> +	fpstate->xfeatures	= fpu_guest_cfg.default_features;
> +	fpstate->user_size	= fpu_user_cfg.default_size;
> +	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
> +	fpstate->xfd		= 0;
>  
> -	/* Leave xfd to 0 (the reset value defined by spec) */
> -	__fpstate_reset(fpstate, 0);
>  	fpstate_init_user(fpstate);
>  	fpstate->is_valloc	= true;
>  	fpstate->is_guest	= true;
>  
>  	gfpu->fpstate		= fpstate;
> -	gfpu->xfeatures		= fpu_user_cfg.default_features;
> -	gfpu->perm		= fpu_user_cfg.default_features;
> +	gfpu->xfeatures		= fpu_guest_cfg.default_features;
> +	gfpu->perm		= fpu_guest_cfg.default_features;
> +
> +	return fpstate;
> +}
> +
> +bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
> +{
> +	struct fpstate *fpstate;
> +
> +	fpstate = __fpu_alloc_init_guest_fpstate(gfpu);
> +
> +	if (!fpstate)
> +		return false;
>  
>  	/*
>  	 * KVM sets the FP+SSE bits in the XSAVE header when copying FPU state

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky