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

Sean Christopherson
2025-05-07T18:01:21Z

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