Re: [PATCH v6 5/7] x86/fpu: Initialize guest fpstate and FPU pseudo container from guest defaults

[Sean Christopherson <seanjc@google.com>]

On Wed, May 07, 2025, Chao Gao wrote:
> From: Chao Gao <chao.gao@intel.com>
> Date: Fri, 31 May 2024 02:03:30 -0700
> Subject: [PATCH] x86/fpu: Initialize guest fpstate and FPU pseudo container
>  from guest defaults
> 
> fpu_alloc_guest_fpstate() currently uses host defaults to initialize guest
> fpstate and pseudo containers. Guest defaults were introduced to
> differentiate the features and sizes of host and guest FPUs. Switch to
> using guest defaults instead.
> 
> Adjust __fpstate_reset() to handle different defaults for host and guest
> FPUs. And to distinguish between the types of FPUs, move the initialization
> of indicators (is_guest and is_valloc) before the reset.
> 
> Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>  arch/x86/kernel/fpu/core.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 444e517a8648..0d501bd25d79 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -236,19 +236,22 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>  	struct fpstate *fpstate;
>  	unsigned int size;
>  
> -	size = fpu_kernel_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
> +	size = guest_default_cfg.size + ALIGN(offsetof(struct fpstate, regs), 64);
> +
>  	fpstate = vzalloc(size);
>  	if (!fpstate)
>  		return false;
>  
> +	/* Initialize indicators to reflect properties of the fpstate */
> +	fpstate->is_valloc	= true;
> +	fpstate->is_guest	= true;
> +
>  	/* 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_kernel_cfg.default_features;
> +	gfpu->xfeatures		= guest_default_cfg.features;
>  
>  	/*
>  	 * KVM sets the FP+SSE bits in the XSAVE header when copying FPU state
> @@ -535,10 +538,20 @@ void fpstate_init_user(struct fpstate *fpstate)
>  
>  static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
>  {
> -	/* Initialize sizes and feature masks */
> -	fpstate->size		= fpu_kernel_cfg.default_size;
> +	/*
> +	 * Initialize sizes and feature masks. Supervisor features and
> +	 * sizes may diverge between guest FPUs and host FPUs, whereas
> +	 * user features and sizes are always identical the same.
> +	 */
> +	if (fpstate->is_guest) {
> +		fpstate->size		= guest_default_cfg.size;
> +		fpstate->xfeatures	= guest_default_cfg.features;
> +	} else {
> +		fpstate->size		= fpu_kernel_cfg.default_size;
> +		fpstate->xfeatures	= fpu_kernel_cfg.default_features;
> +	}

Nice!  I like this idea.

> +
>  	fpstate->user_size	= fpu_user_cfg.default_size;
> -	fpstate->xfeatures	= fpu_kernel_cfg.default_features;
>  	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
>  	fpstate->xfd		= xfd;

And then a follow-up patch (or same patch?) to do this?

	if (fpstate->is_guest) {
		fpstate->size           = guest_default_cfg.size;
		fpstate->xfeatures      = guest_default_cfg.features;
		fpstate->xfd		= 0;
	} else {
		fpstate->size           = fpu_kernel_cfg.default_size;
		fpstate->xfeatures      = fpu_kernel_cfg.default_features;
		fpstate->xfd		= init_fpstate.xfd;
	}
>  }
> -- 
> 2.47.1