Remove @perm from the guest pseudo FPU container. The field is
initialized during allocation and never used later.

Rename fpu_init_guest_permissions() to show that its sole purpose is to
lock down guest permissions.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
v5: drop the useless fpu_guest argument (Chang)
---
 arch/x86/include/asm/fpu/types.h | 7 -------
 arch/x86/kernel/fpu/core.c       | 7 ++-----
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 46cc263f9f4f..9f9ed406b179 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -526,13 +526,6 @@ struct fpu_guest {
 	 */
 	u64				xfeatures;
 
-	/*
-	 * @perm:			xfeature bitmap of features which are
-	 *				permitted to be enabled for the guest
-	 *				vCPU.
-	 */
-	u64				perm;
-
 	/*
 	 * @xfd_err:			Save the guest value.
 	 */
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1b734a9ff088..28ad7ec56eaa 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -202,7 +202,7 @@ 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)
+static void fpu_lock_guest_permissions(void)
 {
 	struct fpu_state_perm *fpuperm;
 	u64 perm;
@@ -218,8 +218,6 @@ static void fpu_init_guest_permissions(struct fpu_guest *gfpu)
 	WRITE_ONCE(fpuperm->__state_perm, perm | FPU_GUEST_PERM_LOCKED);
 
 	spin_unlock_irq(&current->sighand->siglock);
-
-	gfpu->perm = perm & ~FPU_GUEST_PERM_LOCKED;
 }
 
 bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
@@ -240,7 +238,6 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
 
 	gfpu->fpstate		= fpstate;
 	gfpu->xfeatures		= fpu_kernel_cfg.default_features;
-	gfpu->perm		= fpu_kernel_cfg.default_features;
 
 	/*
 	 * KVM sets the FP+SSE bits in the XSAVE header when copying FPU state
@@ -255,7 +252,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
 	if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
 		gfpu->uabi_size = fpu_user_cfg.default_size;
 
-	fpu_init_guest_permissions(gfpu);
+	fpu_lock_guest_permissions();
 
 	return true;
 }
-- 
2.46.1

On 4/10/2025 12:24 AM, Chao Gao wrote:
> Remove @perm from the guest pseudo FPU container. The field is
> initialized during allocation and never used later.
> 
> Rename fpu_init_guest_permissions() to show that its sole purpose is to
> lock down guest permissions.
> 
> Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>

This patch appears to be new in V3, as I can see from the diff here:

https://github.com/ChangSeokBae/kernel/compare/xstate-scet-chao-v2...xstate-scet-chao-v3

However, I don’t see any relevant comment from Maxim on your V2 series. 
Unlike patch 1, this one doesn’t include a URL referencing the 
suggestion either -- so I suspect the Suggested-by tag might be incorrect.
> @@ -255,7 +252,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>   	if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
>   		gfpu->uabi_size = fpu_user_cfg.default_size;
>   
> -	fpu_init_guest_permissions(gfpu);
> +	fpu_lock_guest_permissions();

As a future improvement, you might consider updating this to:

     if (xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED)
         fpu_lock_guest_permissions();

Or, embed the check inside fpu_lock_guest_permissions():

     if (xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED)
         return;

But for this patch itself, the change looks good to me. Please feel free 
to add my tag:

     Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>

Thanks,
Chang

On Fri, Apr 18, 2025 at 01:51:02PM -0700, Chang S. Bae wrote:
>On 4/10/2025 12:24 AM, Chao Gao wrote:
>> Remove @perm from the guest pseudo FPU container. The field is
>> initialized during allocation and never used later.
>> 
>> Rename fpu_init_guest_permissions() to show that its sole purpose is to
>> lock down guest permissions.
>> 
>> Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
>
>This patch appears to be new in V3, as I can see from the diff here:
>
>https://github.com/ChangSeokBae/kernel/compare/xstate-scet-chao-v2...xstate-scet-chao-v3
>
>However, I don’t see any relevant comment from Maxim on your V2 series.
>Unlike patch 1, this one doesn’t include a URL referencing the suggestion
>either -- so I suspect the Suggested-by tag might be incorrect.

v3 was the version where I truly began refining the patches based on my
understanding, the historical discussion, and feedback on v2 [*]. While
reviewing the historical discussion, I found Maxim's suggestion to be
valuable:

https://lore.kernel.org/kvm/af972fe5981b9e7101b64de43c7be0a8cc165323.camel@redhat.com/

So, I implemented it in v3, but I should have included the link.

[*] v2 was simply a resend of v1
    https://lore.kernel.org/kvm/20241126101710.62492-1-chao.gao@intel.com/

>> @@ -255,7 +252,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>>   	if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
>>   		gfpu->uabi_size = fpu_user_cfg.default_size;
>> -	fpu_init_guest_permissions(gfpu);
>> +	fpu_lock_guest_permissions();
>
>As a future improvement, you might consider updating this to:
>
>    if (xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED)
>        fpu_lock_guest_permissions();
>
>Or, embed the check inside fpu_lock_guest_permissions():
>
>    if (xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED)
>        return;
>
>But for this patch itself, the change looks good to me. Please feel free to
>add my tag:
>
>    Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>

Thanks a lot.

On 4/18/2025 1:51 PM, Chang S. Bae wrote:
>      if (xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED)
>          fpu_lock_guest_permissions();

Sorry, this should be:

     if (!(xstate_get_guest_group_perm() & FPU_GUEST_PERM_LOCKED))
         fpu_lock_guest_permissions();