On Mon, May 12, 2025, Chao Gao wrote:
> @@ -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.
Pick of of "identical" or "the same" :-)
And maybe explain why supervisor features can diverge, while the kernel ensures
user features are identical? Ditto for the XFD divergence. E.g. I think this
would be accurate (though I may be reading too much into user features):
/*
* Supervisor features (and thus sizes) may diverge between guest FPUs
* and host FPUs, as some supervisor features are supported for guests
* despite not being utilized by the host. User features and sizes are
* always identical, which allows for common guest and userspace ABI.
*
* For the host, set XFD to the kernel's desired initialization value.
* For guests, set XFD to its architectural RESET value.
*/
> + */
> + 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;
> + }
> +
> 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;
> }
> --
> 2.47.1
>
