Re: [PATCH 05/19] x86/fpu: Move xfd initialization out of __fpstate_reset() to the callers

[Thomas Gleixner <tglx@linutronix.de>]

Yang, Jing,

On Tue, Dec 07 2021 at 19:03, Yang Zhong wrote:
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index fe592799508c..fae44fa27cdb 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -231,6 +231,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>  	if (!fpstate)
>  		return false;
>  
> +	/* Leave xfd to 0 (the reset value defined by spec) */
>  	__fpstate_reset(fpstate);

That change makes me a bit wary simply because the comment here is above
__fpstate_reset() which makes no sense. It does make sense to you at the
time, but does it make sense to you when you look at it 6 month down the
road?

So I'd rather make this very obvious what's going. See below.

Thanks,

        tglx
---

--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -199,7 +199,7 @@ void fpu_reset_from_exception_fixup(void
 }
 
 #if IS_ENABLED(CONFIG_KVM)
-static void __fpstate_reset(struct fpstate *fpstate);
+static void __fpstate_reset(struct fpstate *fpstate, u64 xfd);
 
 static void fpu_init_guest_permissions(struct fpu_guest *gfpu)
 {
@@ -231,7 +231,8 @@ bool fpu_alloc_guest_fpstate(struct fpu_
 	if (!fpstate)
 		return false;
 
-	__fpstate_reset(fpstate);
+	/* 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;
@@ -454,21 +455,21 @@ void fpstate_init_user(struct fpstate *f
 		fpstate_init_fstate(fpstate);
 }
 
-static void __fpstate_reset(struct fpstate *fpstate)
+static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
 {
 	/* Initialize sizes and feature masks */
 	fpstate->size		= fpu_kernel_cfg.default_size;
 	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		= init_fpstate.xfd;
+	fpstate->xfd		= xfd;
 }
 
 void fpstate_reset(struct fpu *fpu)
 {
 	/* Set the fpstate pointer to the default fpstate */
 	fpu->fpstate = &fpu->__fpstate;
-	__fpstate_reset(fpu->fpstate);
+	__fpstate_reset(fpu->fpstate, init_fpstate.xfd);
 
 	/* Initialize the permission related info in fpu */
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;