[PATCH v6 0/7] Introduce CET supervisor state support

[PATCH v6 0/7] Introduce CET supervisor state support

[Chao Gao]

Dear maintainers and reviewers,

I kindly request your consideration for merging this series. The changes
between v5 (posted one month ago) and v6 are minimal, and most of the
patches have received Reviewed-by/Acked-by tags.

Thanks Chang, Rick, Xin and Sean for their help with this series.

== Changelog ==
v5->v6:
 - Collect reviews from Chang and Rick
 - Reset guest default size to FPU legacy size when XSAVE is not available
   or not enabled (Chang)
 - Drop guest default user features and size, as they will not differ from
   host FPUs (Rick)
 - v5: https://lore.kernel.org/kvm/20250410072605.2358393-1-chao.gao@intel.com/

== Background ==

CET defines two register states: CET user, which includes user-mode control
registers, and CET supervisor, which consists of shadow-stack pointers for
privilege levels 0-2.

Current kernels disable shadow stacks in kernel mode, making the CET
supervisor state unused and eliminating the need for context switching.

== Problem ==

To virtualize CET for guests, KVM must accurately emulate hardware
behavior. A key challenge arises because there is no CPUID flag to indicate
that shadow stack is supported only in user mode. Therefore, KVM cannot
assume guests will not enable shadow stacks in kernel mode and must
preserve the CET supervisor state of vCPUs.

== Solution ==

An initial proposal to manually save and restore CET supervisor states
using raw RDMSR/WRMSR in KVM was rejected due to performance concerns and
its impact on KVM's ABI. Instead, leveraging the kernel's FPU
infrastructure for context switching was favored [1].

The main question then became whether to enable the CET supervisor state
globally for all processes or restrict it to vCPU processes. This decision
involves a trade-off between a 24-byte XSTATE buffer waste for all non-vCPU
processes and approximately 100 lines of code complexity in the kernel [2].
The agreed approach is to first try this optimal solution [3], i.e.,
restricting the CET supervisor state to guest FPUs only and eliminating
unnecessary space waste.

Key changes in this series are:

1) Fix existing issue regarding enabling guest supervisor states support.
2) Add default features and size for guest FPUs.
3) Add infrastructure to support guest-only features.
4) Add CET supervisor state as the first guest-only feature.

With the series in place, guest FPUs have xstate_bv[12] == xcomp_bv[12] == 1
and CET supervisor state is saved/reloaded when xsaves/xrstors executes on
guest FPUs. non-guest FPUs have xstate_bv[12] == xcomp_bv[12] == 0, then
CET supervisor state is not saved/restored.

== Performance ==

We measured context-switching performance with the benchmark [4] in following
three cases.

case 1: the baseline. i.e., this series isn't applied
case 2: baseline + this series. CET-S space is allocated for guest fpu only.
case 3: baseline + allocate CET-S space for all tasks. Hardware init
        optimization avoids writing out CET-S space on each XSAVES.

The performance differences in the three cases are very small and fall within the
run-to-run variation.

[1]: https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@google.com/
[2]: https://lore.kernel.org/kvm/1c2fd06e-2e97-4724-80ab-8695aa4334e7@intel.com/
[3]: https://lore.kernel.org/kvm/2597a87b-1248-b8ce-ce60-94074bc67ea4@intel.com/
[4]: https://github.com/antonblanchard/will-it-scale/blob/master/tests/context_switch1.c

Chao Gao (4):
  x86/fpu: Drop @perm from guest pseudo FPU container
  x86/fpu/xstate: Differentiate default features for host and guest FPUs
  x86/fpu: Initialize guest FPU permissions from guest defaults
  x86/fpu: Initialize guest fpstate and FPU pseudo container from guest
    defaults

Sean Christopherson (1):
  x86/fpu/xstate: Always preserve non-user xfeatures/flags in
    __state_perm

Yang Weijiang (2):
  x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set
  x86/fpu/xstate: Add CET supervisor xfeature support as a guest-only
    feature

 arch/x86/include/asm/fpu/types.h  | 64 +++++++++++++++++++++++--------
 arch/x86/include/asm/fpu/xstate.h |  9 +++--
 arch/x86/kernel/fpu/core.c        | 46 +++++++++++++++-------
 arch/x86/kernel/fpu/init.c        |  1 +
 arch/x86/kernel/fpu/xstate.c      | 58 +++++++++++++++++++++-------
 arch/x86/kernel/fpu/xstate.h      |  5 +++
 6 files changed, 137 insertions(+), 46 deletions(-)


base-commit: 960bc2bcba5987a82530b9756e1f602a894cffa4
-- 
2.47.1

[PATCH v6 1/7] x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state_perm

[Chao Gao]

From: Sean Christopherson <seanjc@google.com>

When granting userspace or a KVM guest access to an xfeature, preserve the
entity's existing supervisor and software-defined permissions as tracked
by __state_perm, i.e. use __state_perm to track *all* permissions even
though all supported supervisor xfeatures are granted to all FPUs and
FPU_GUEST_PERM_LOCKED disallows changing permissions.

Effectively clobbering supervisor permissions results in inconsistent
behavior, as xstate_get_group_perm() will report supervisor features for
process that do NOT request access to dynamic user xfeatures, whereas any
and all supervisor features will be absent from the set of permissions for
any process that is granted access to one or more dynamic xfeatures (which
right now means AMX).

The inconsistency isn't problematic because fpu_xstate_prctl() already
strips out everything except user xfeatures:

        case ARCH_GET_XCOMP_PERM:
                /*
                 * Lockless snapshot as it can also change right after the
                 * dropping the lock.
                 */
                permitted = xstate_get_host_group_perm();
                permitted &= XFEATURE_MASK_USER_SUPPORTED;
                return put_user(permitted, uptr);

        case ARCH_GET_XCOMP_GUEST_PERM:
                permitted = xstate_get_guest_group_perm();
                permitted &= XFEATURE_MASK_USER_SUPPORTED;
                return put_user(permitted, uptr);

and similarly KVM doesn't apply the __state_perm to supervisor states
(kvm_get_filtered_xcr0() incorporates xstate_get_guest_group_perm()):

        case 0xd: {
                u64 permitted_xcr0 = kvm_get_filtered_xcr0();
                u64 permitted_xss = kvm_caps.supported_xss;

But if KVM in particular were to ever change, dropping supervisor
permissions would result in subtle bugs in KVM's reporting of supported
CPUID settings.  And the above behavior also means that having supervisor
xfeatures in __state_perm is correctly handled by all users.

Dropping supervisor permissions also creates another landmine for KVM.  If
more dynamic user xfeatures are ever added, requesting access to multiple
xfeatures in separate ARCH_REQ_XCOMP_GUEST_PERM calls will result in the
second invocation of __xstate_request_perm() computing the wrong ksize, as
as the mask passed to xstate_calculate_size() would not contain *any*
supervisor features.

Commit 781c64bfcb73 ("x86/fpu/xstate: Handle supervisor states in XSTATE
permissions") fudged around the size issue for userspace FPUs, but for
reasons unknown skipped guest FPUs.  Lack of a fix for KVM "works" only
because KVM doesn't yet support virtualizing features that have supervisor
xfeatures, i.e. as of today, KVM guest FPUs will never need the relevant
xfeatures.

Simply extending the hack-a-fix for guests would temporarily solve the
ksize issue, but wouldn't address the inconsistency issue and would leave
another lurking pitfall for KVM.  KVM support for virtualizing CET will
likely add CET_KERNEL as a guest-only xfeature, i.e. CET_KERNEL will not
be set in xfeatures_mask_supervisor() and would again be dropped when
granting access to dynamic xfeatures.

Note, the existing clobbering behavior is rather subtle.  The @permitted
parameter to __xstate_request_perm() comes from:

	permitted = xstate_get_group_perm(guest);

which is either fpu->guest_perm.__state_perm or fpu->perm.__state_perm,
where __state_perm is initialized to:

        fpu->perm.__state_perm          = fpu_kernel_cfg.default_features;

and copied to the guest side of things:

	/* Same defaults for guests */
	fpu->guest_perm = fpu->perm;

fpu_kernel_cfg.default_features contains everything except the dynamic
xfeatures, i.e. everything except XFEATURE_MASK_XTILE_DATA:

        fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features;
        fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;

When __xstate_request_perm() restricts the local "mask" variable to
compute the user state size:

	mask &= XFEATURE_MASK_USER_SUPPORTED;
	usize = xstate_calculate_size(mask, false);

it subtly overwrites the target __state_perm with "mask" containing only
user xfeatures:

	perm = guest ? &fpu->guest_perm : &fpu->perm;
	/* Pairs with the READ_ONCE() in xstate_get_group_perm() */
	WRITE_ONCE(perm->__state_perm, mask);

Cc: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Weijiang Yang <weijiang.yang@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Chao Gao <chao.gao@intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: John Allen <john.allen@amd.com>
Cc: kvm@vger.kernel.org
Link: https://lore.kernel.org/all/ZTqgzZl-reO1m01I@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
---
 arch/x86/include/asm/fpu/types.h |  8 +++++---
 arch/x86/kernel/fpu/xstate.c     | 18 +++++++++++-------
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 97310df3ea13..e64db0eb9d27 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -416,9 +416,11 @@ struct fpu_state_perm {
 	/*
 	 * @__state_perm:
 	 *
-	 * This bitmap indicates the permission for state components, which
-	 * are available to a thread group. The permission prctl() sets the
-	 * enabled state bits in thread_group_leader()->thread.fpu.
+	 * This bitmap indicates the permission for state components
+	 * available to a thread group, including both user and supervisor
+	 * components and software-defined bits like FPU_GUEST_PERM_LOCKED.
+	 * The permission prctl() sets the enabled state bits in
+	 * thread_group_leader()->thread.fpu.
 	 *
 	 * All run time operations use the per thread information in the
 	 * currently active fpu.fpstate which contains the xfeature masks
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 8b14c9d3a1df..1c8410b68108 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1656,16 +1656,20 @@ static int __xstate_request_perm(u64 permitted, u64 requested, bool guest)
 	if ((permitted & requested) == requested)
 		return 0;
 
-	/* Calculate the resulting kernel state size */
+	/*
+	 * Calculate the resulting kernel state size.  Note, @permitted also
+	 * contains supervisor xfeatures even though supervisor are always
+	 * permitted for kernel and guest FPUs, and never permitted for user
+	 * FPUs.
+	 */
 	mask = permitted | requested;
-	/* Take supervisor states into account on the host */
-	if (!guest)
-		mask |= xfeatures_mask_supervisor();
 	ksize = xstate_calculate_size(mask, compacted);
 
-	/* Calculate the resulting user state size */
-	mask &= XFEATURE_MASK_USER_SUPPORTED;
-	usize = xstate_calculate_size(mask, false);
+	/*
+	 * Calculate the resulting user state size.  Take care not to clobber
+	 * the supervisor xfeatures in the new mask!
+	 */
+	usize = xstate_calculate_size(mask & XFEATURE_MASK_USER_SUPPORTED, false);
 
 	if (!guest) {
 		ret = validate_sigaltstack(usize);
-- 
2.47.1

[PATCH v6 2/7] x86/fpu: Drop @perm from guest pseudo FPU container

[Chao Gao]

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>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Link: https://lore.kernel.org/kvm/af972fe5981b9e7101b64de43c7be0a8cc165323.camel@redhat.com/
---
v6: add Chang's Reviewed-by
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 e64db0eb9d27..1c94121acd3d 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -535,13 +535,6 @@ 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 105b1b80d88d..1cda5b78540b 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -212,7 +212,7 @@ 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;
@@ -228,8 +228,6 @@ 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)
@@ -250,7 +248,6 @@ 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
@@ -265,7 +262,7 @@ 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;
 }
-- 
2.47.1

[PATCH v6 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

[Chao Gao]

Currently, guest and host FPUs share the same default features. However,
the CET supervisor xstate is the first feature that needs to be enabled
exclusively for guest FPUs. Enabling it for host FPUs leads to a waste of
24 bytes in the XSAVE buffer.

To support "guest-only" features, add a new structure to hold the
default features and sizes for guest FPUs to clearly differentiate them
from those for host FPUs.

Note that,
1) for now, the default features for guest and host FPUs remain the
same. This will change in a follow-up patch once guest permissions, default
xfeatures, and fpstate size are all converted to use the guest defaults.

2) only supervisor features will diverge between guest FPUs and host
FPUs, while user features will remain the same [1][2]. So, the new
vcpu_fpu_config struct does not include default user features and size
for the UABI buffer.

An alternative approach is adding a guest_only_xfeatures member to
fpu_kernel_cfg and adding two helper functions to calculate the guest
default xfeatures and size. However, calculating these defaults at runtime
would introduce unnecessary overhead.

Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/kvm/aAwdQ759Y6V7SGhv@google.com/ [1]
Link: https://lore.kernel.org/kvm/9ca17e1169805f35168eb722734fbf3579187886.camel@intel.com/ [2]
---
v6:
Drop vcpu_fpu_config.user_* (Rick)
Reset guest default size when XSAVE is unavaiable or disabled (Chang)

v5:
Add a new vcpu_fpu_config instead of adding new members to
fpu_state_config (Chang)
Extract a helper to set default values (Chang)
---
 arch/x86/include/asm/fpu/types.h | 26 ++++++++++++++++++++++++++
 arch/x86/kernel/fpu/core.c       |  1 +
 arch/x86/kernel/fpu/init.c       |  1 +
 arch/x86/kernel/fpu/xstate.c     | 27 +++++++++++++++++++++------
 4 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 1c94121acd3d..abd193a1a52e 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -551,6 +551,31 @@ struct fpu_guest {
 	struct fpstate			*fpstate;
 };
 
+/*
+ * FPU state configuration data for fpu_guest.
+ * Initialized at boot time. Read only after init.
+ */
+struct vcpu_fpu_config {
+	/*
+	 * @size:
+	 *
+	 * The default size of the register state buffer in guest FPUs.
+	 * Includes all supported features except independent managed
+	 * features and features which have to be requested by user space
+	 * before usage.
+	 */
+	unsigned int size;
+
+	/*
+	 * @features:
+	 *
+	 * The default supported features bitmap in guest FPUs. Does not
+	 * include independent managed features and features which have to
+	 * be requested by user space before usage.
+	 */
+	u64 features;
+};
+
 /*
  * FPU state configuration data. Initialized at boot time. Read only after init.
  */
@@ -606,5 +631,6 @@ struct fpu_state_config {
 
 /* FPU state configuration information */
 extern struct fpu_state_config fpu_kernel_cfg, fpu_user_cfg;
+extern struct vcpu_fpu_config guest_default_cfg;
 
 #endif /* _ASM_X86_FPU_TYPES_H */
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1cda5b78540b..2cd5e1910ff8 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -36,6 +36,7 @@ DEFINE_PER_CPU(u64, xfd_state);
 /* The FPU state configuration data for kernel and user space */
 struct fpu_state_config	fpu_kernel_cfg __ro_after_init;
 struct fpu_state_config fpu_user_cfg __ro_after_init;
+struct vcpu_fpu_config guest_default_cfg __ro_after_init;
 
 /*
  * Represents the initial FPU state. It's mostly (but not completely) zeroes,
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 6bb3e35c40e2..e19660cdc70c 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -202,6 +202,7 @@ static void __init fpu__init_system_xstate_size_legacy(void)
 	fpu_kernel_cfg.default_size = size;
 	fpu_user_cfg.max_size = size;
 	fpu_user_cfg.default_size = size;
+	guest_default_cfg.size = size;
 }
 
 /*
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 1c8410b68108..f32047e12500 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -742,6 +742,9 @@ static int __init init_xstate_size(void)
 	fpu_user_cfg.default_size =
 		xstate_calculate_size(fpu_user_cfg.default_features, false);
 
+	guest_default_cfg.size =
+		xstate_calculate_size(guest_default_cfg.features, compacted);
+
 	return 0;
 }
 
@@ -762,6 +765,7 @@ static void __init fpu__init_disable_system_xstate(unsigned int legacy_size)
 	fpu_kernel_cfg.default_size = legacy_size;
 	fpu_user_cfg.max_size = legacy_size;
 	fpu_user_cfg.default_size = legacy_size;
+	guest_default_cfg.size = legacy_size;
 
 	/*
 	 * Prevent enabling the static branch which enables writes to the
@@ -772,6 +776,21 @@ static void __init fpu__init_disable_system_xstate(unsigned int legacy_size)
 	fpstate_reset(x86_task_fpu(current));
 }
 
+static void __init init_default_features(u64 kernel_max_features, u64 user_max_features)
+{
+	u64 kfeatures = kernel_max_features;
+	u64 ufeatures = user_max_features;
+
+	/* Default feature sets should not include dynamic xfeatures. */
+	kfeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
+	ufeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
+
+	fpu_kernel_cfg.default_features = kfeatures;
+	fpu_user_cfg.default_features   = ufeatures;
+
+	guest_default_cfg.features      = kfeatures;
+}
+
 /*
  * Enable and initialize the xsave feature.
  * Called once per system bootup.
@@ -854,12 +873,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	fpu_user_cfg.max_features = fpu_kernel_cfg.max_features;
 	fpu_user_cfg.max_features &= XFEATURE_MASK_USER_SUPPORTED;
 
-	/* Clean out dynamic features from default */
-	fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features;
-	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
-
-	fpu_user_cfg.default_features = fpu_user_cfg.max_features;
-	fpu_user_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
+	/* Now, given maximum feature set, determine default values */
+	init_default_features(fpu_kernel_cfg.max_features, fpu_user_cfg.max_features);
 
 	/* Store it for paranoia check at the end */
 	xfeatures = fpu_kernel_cfg.max_features;
-- 
2.47.1

[PATCH v6 4/7] x86/fpu: Initialize guest FPU permissions from guest defaults

[Chao Gao]

Currently, fpu->guest_perm is copied from fpu->perm, which is derived from
fpu_kernel_cfg.default_features.

Guest defaults were introduced to differentiate the features and sizes of
host and guest FPUs. Copying guest FPU permissions from the host will lead
to inconsistencies between the guest default features and permissions.

Initialize guest FPU permissions from guest defaults instead of host
defaults. This ensures that any changes to guest default features are
automatically reflected in guest permissions, which in turn guarantees
that fpstate_realloc() allocates a correctly sized XSAVE buffer for guest
FPUs.

Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
v6: Drop vcpu_fpu_config.user_* and collect reviews (Rick)
---
 arch/x86/kernel/fpu/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 2cd5e1910ff8..444e517a8648 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -553,8 +553,14 @@ void fpstate_reset(struct fpu *fpu)
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
 	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
 	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
-	/* Same defaults for guests */
-	fpu->guest_perm = fpu->perm;
+
+	fpu->guest_perm.__state_perm	= guest_default_cfg.features;
+	fpu->guest_perm.__state_size	= guest_default_cfg.size;
+	/*
+	 * User features and sizes remain the same between guest FPUs
+	 * and host FPUs.
+	 */
+	fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
 }
 
 static inline void fpu_inherit_perms(struct fpu *dst_fpu)
-- 
2.47.1

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

[Chao Gao]

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.

Additionally, incorporate the initialization of indicators (is_valloc and
is_guest) into the newly added guest-specific reset function to centralize
the resetting of guest fpstate.

Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
---
v6: Drop vcpu_fpu_config.user_* (Rick)
v5: init is_valloc/is_guest in the guest-specific reset function (Chang)
---
 arch/x86/kernel/fpu/core.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 444e517a8648..78a9c809dfad 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -211,7 +211,24 @@ void fpu_reset_from_exception_fixup(void)
 }
 
 #if IS_ENABLED(CONFIG_KVM)
-static void __fpstate_reset(struct fpstate *fpstate, u64 xfd);
+static void __guest_fpstate_reset(struct fpstate *fpstate, u64 xfd)
+{
+	/*
+	 * Initialize sizes and feature masks. Supervisor features and
+	 * sizes may diverge between guest FPUs and host FPUs, whereas
+	 * user features and sizes remain the same.
+	 */
+	fpstate->size		= guest_default_cfg.size;
+	fpstate->xfeatures	= guest_default_cfg.features;
+	fpstate->user_size	= fpu_user_cfg.default_size;
+	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
+	fpstate->xfd		= xfd;
+
+	/* Initialize indicators to reflect properties of the fpstate */
+	fpstate->is_valloc	= true;
+	fpstate->is_guest	= true;
+}
+
 
 static void fpu_lock_guest_permissions(void)
 {
@@ -236,19 +253,18 @@ 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;
 
 	/* Leave xfd to 0 (the reset value defined by spec) */
-	__fpstate_reset(fpstate, 0);
+	__guest_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
-- 
2.47.1

[PATCH v6 6/7] x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

In preparation for upcoming CET virtualization support, the CET supervisor
state will be added as a "guest-only" feature, since it is required only by
KVM (i.e., guest FPUs). Establish the infrastructure for "guest-only"
features.

Define a new XFEATURE_MASK_GUEST_SUPERVISOR mask to specify features that
are enabled by default in guest FPUs but not in host FPUs. Specifically,
for any bit in this set, permission is granted and XSAVE space is allocated
during vCPU creation. Non-guest FPUs cannot enable guest-only features,
even dynamically, and no XSAVE space will be allocated for them.

The mask is currently empty, but this will be changed by a subsequent
patch.

Co-developed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
v6: Collect reviews

v5: Explain in detail the reasoning behind the mask name choice below the
"---" separator line.

In previous versions, the mask was named "XFEATURE_MASK_SUPERVISOR_DYNAMIC"
Dave suggested this name [1], but he also noted, "I don't feel strongly about
it and I've said my piece. I won't NAK it one way or the other."

The term "dynamic" was initially preferred because it reflects the impact
on XSAVE buffers—some buffers accommodate dynamic features while others do
not. This naming allows for the introduction of dynamic features that are
not strictly "guest-only", offering flexibility beyond KVM.

However, using "dynamic" has led to confusion [2]. Chang pointed out that
permission granting and buffer allocation are actually static at VCPU
allocation, diverging from the model for user dynamic features. He also
questioned the rationale for introducing a kernel dynamic feature mask
while using it as a guest-only feature mask [3]. Moreover, Thomas remarked
that "the dynamic naming is really bad" [4]. Although his specific concerns
are unclear, we should be cautious about reinstating the "kernel dynamic
feature" naming.

Therefore, in v4, I renamed the mask to "XFEATURE_MASK_SUPERVISOR_GUEST"
and further refined it to "XFEATURE_MASK_GUEST_SUPERVISOR" in this v5.

[1]: https://lore.kernel.org/all/893ac578-baaf-4f4f-96ee-e012dfc073a8@intel.com/#t
[2]: https://lore.kernel.org/kvm/e15d1074-d5ec-431d-86e5-a58bc6297df8@intel.com/
[3]: https://lore.kernel.org/kvm/7bee70fd-b2b9-4466-a694-4bf3486b19c7@intel.com/
[4]: https://lore.kernel.org/all/87sg1owmth.ffs@nanos.tec.linutronix.de/
---
 arch/x86/include/asm/fpu/types.h  |  9 +++++----
 arch/x86/include/asm/fpu/xstate.h |  6 +++++-
 arch/x86/kernel/fpu/xstate.c      | 14 +++++++++++---
 arch/x86/kernel/fpu/xstate.h      |  5 +++++
 4 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index abd193a1a52e..54ba567258d6 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -592,8 +592,9 @@ struct fpu_state_config {
 	 * @default_size:
 	 *
 	 * The default size of the register state buffer. Includes all
-	 * supported features except independent managed features and
-	 * features which have to be requested by user space before usage.
+	 * supported features except independent managed features,
+	 * guest-only features and features which have to be requested by
+	 * user space before usage.
 	 */
 	unsigned int		default_size;
 
@@ -609,8 +610,8 @@ struct fpu_state_config {
 	 * @default_features:
 	 *
 	 * The default supported features bitmap. Does not include
-	 * independent managed features and features which have to
-	 * be requested by user space before usage.
+	 * independent managed features, guest-only features and features
+	 * which have to be requested by user space before usage.
 	 */
 	u64 default_features;
 	/*
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index b308a76afbb7..a3cd25453f94 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -46,9 +46,13 @@
 /* Features which are dynamically enabled for a process on request */
 #define XFEATURE_MASK_USER_DYNAMIC	XFEATURE_MASK_XTILE_DATA
 
+/* Supervisor features which are enabled only in guest FPUs */
+#define XFEATURE_MASK_GUEST_SUPERVISOR	0
+
 /* All currently supported supervisor features */
 #define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \
-					    XFEATURE_MASK_CET_USER)
+					    XFEATURE_MASK_CET_USER | \
+					    XFEATURE_MASK_GUEST_SUPERVISOR)
 
 /*
  * A supervisor state component may not always contain valuable information,
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index f32047e12500..e77cbfd18094 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -781,14 +781,22 @@ static void __init init_default_features(u64 kernel_max_features, u64 user_max_f
 	u64 kfeatures = kernel_max_features;
 	u64 ufeatures = user_max_features;
 
-	/* Default feature sets should not include dynamic xfeatures. */
-	kfeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
+	/*
+	 * Default feature sets should not include dynamic and guest-only
+	 * xfeatures at all.
+	 */
+	kfeatures &= ~(XFEATURE_MASK_USER_DYNAMIC | XFEATURE_MASK_GUEST_SUPERVISOR);
 	ufeatures &= ~XFEATURE_MASK_USER_DYNAMIC;
 
 	fpu_kernel_cfg.default_features = kfeatures;
 	fpu_user_cfg.default_features   = ufeatures;
 
-	guest_default_cfg.features      = kfeatures;
+	/*
+	 * Ensure VCPU FPU container only reserves a space for guest-only
+	 * xfeatures. This distinction can save kernel memory by
+	 * maintaining a necessary amount of XSAVE buffer.
+	 */
+	guest_default_cfg.features      = kfeatures | xfeatures_mask_guest_supervisor();
 }
 
 /*
diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index a0256ef34ecb..5ced1a92e666 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -61,6 +61,11 @@ static inline u64 xfeatures_mask_supervisor(void)
 	return fpu_kernel_cfg.max_features & XFEATURE_MASK_SUPERVISOR_SUPPORTED;
 }
 
+static inline u64 xfeatures_mask_guest_supervisor(void)
+{
+	return fpu_kernel_cfg.max_features & XFEATURE_MASK_GUEST_SUPERVISOR;
+}
+
 static inline u64 xfeatures_mask_independent(void)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR))
-- 
2.47.1

[PATCH v6 7/7] x86/fpu/xstate: Add CET supervisor xfeature support as a guest-only feature

[Chao Gao]

From: Yang Weijiang <weijiang.yang@intel.com>

== Background ==

CET defines two register states: CET user, which includes user-mode control
registers, and CET supervisor, which consists of shadow-stack pointers for
privilege levels 0-2.

Current kernels disable shadow stacks in kernel mode, making the CET
supervisor state unused and eliminating the need for context switching.

== Problem ==

To virtualize CET for guests, KVM must accurately emulate hardware
behavior. A key challenge arises because there is no CPUID flag to indicate
that shadow stack is supported only in user mode. Therefore, KVM cannot
assume guests will not enable shadow stacks in kernel mode and must
preserve the CET supervisor state of vCPUs.

== Solution ==

An initial proposal to manually save and restore CET supervisor states
using raw RDMSR/WRMSR in KVM was rejected due to performance concerns and
its impact on KVM's ABI. Instead, leveraging the kernel's FPU
infrastructure for context switching was favored [1].

The main question then became whether to enable the CET supervisor state
globally for all processes or restrict it to vCPU processes. This decision
involves a trade-off between a 24-byte XSTATE buffer waste for all non-vCPU
processes and approximately 100 lines of code complexity in the kernel [2].
The agreed approach is to first try this optimal solution [3], i.e.,
restricting the CET supervisor state to guest FPUs only and eliminating
unnecessary space waste.

The guest-only xfeature infrastructure has already been added. Now,
introduce CET supervisor xstate support as the first guest-only feature
to prepare for the upcoming CET virtualization in KVM.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@google.com/ [1]
Link: https://lore.kernel.org/kvm/1c2fd06e-2e97-4724-80ab-8695aa4334e7@intel.com/ [2]
Link: https://lore.kernel.org/kvm/2597a87b-1248-b8ce-ce60-94074bc67ea4@intel.com/ [3]
---
v5:
Introduce CET supervisor xfeature directly as a guest-only feature, rather
than first introducing it in one patch and then converting it to guest-only
in a subsequent patch. (Chang)
Add new features after cleanups/bug fixes (Chang, Dave, Ingo)
Improve the commit message to follow the suggested
background-problem-solution pattern.
---
 arch/x86/include/asm/fpu/types.h  | 14 ++++++++++++--
 arch/x86/include/asm/fpu/xstate.h |  5 ++---
 arch/x86/kernel/fpu/xstate.c      |  5 ++++-
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 54ba567258d6..93e99d2583d6 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -118,7 +118,7 @@ enum xfeature {
 	XFEATURE_PKRU,
 	XFEATURE_PASID,
 	XFEATURE_CET_USER,
-	XFEATURE_CET_KERNEL_UNUSED,
+	XFEATURE_CET_KERNEL,
 	XFEATURE_RSRVD_COMP_13,
 	XFEATURE_RSRVD_COMP_14,
 	XFEATURE_LBR,
@@ -142,7 +142,7 @@ enum xfeature {
 #define XFEATURE_MASK_PKRU		(1 << XFEATURE_PKRU)
 #define XFEATURE_MASK_PASID		(1 << XFEATURE_PASID)
 #define XFEATURE_MASK_CET_USER		(1 << XFEATURE_CET_USER)
-#define XFEATURE_MASK_CET_KERNEL	(1 << XFEATURE_CET_KERNEL_UNUSED)
+#define XFEATURE_MASK_CET_KERNEL	(1 << XFEATURE_CET_KERNEL)
 #define XFEATURE_MASK_LBR		(1 << XFEATURE_LBR)
 #define XFEATURE_MASK_XTILE_CFG		(1 << XFEATURE_XTILE_CFG)
 #define XFEATURE_MASK_XTILE_DATA	(1 << XFEATURE_XTILE_DATA)
@@ -268,6 +268,16 @@ struct cet_user_state {
 	u64 user_ssp;
 };
 
+/*
+ * State component 12 is Control-flow Enforcement supervisor states.
+ * This state includes SSP pointers for privilege levels 0 through 2.
+ */
+struct cet_supervisor_state {
+	u64 pl0_ssp;
+	u64 pl1_ssp;
+	u64 pl2_ssp;
+} __packed;
+
 /*
  * State component 15: Architectural LBR configuration state.
  * The size of Arch LBR state depends on the number of LBRs (lbr_depth).
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index a3cd25453f94..7a7dc9d56027 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -47,7 +47,7 @@
 #define XFEATURE_MASK_USER_DYNAMIC	XFEATURE_MASK_XTILE_DATA
 
 /* Supervisor features which are enabled only in guest FPUs */
-#define XFEATURE_MASK_GUEST_SUPERVISOR	0
+#define XFEATURE_MASK_GUEST_SUPERVISOR	XFEATURE_MASK_CET_KERNEL
 
 /* All currently supported supervisor features */
 #define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \
@@ -79,8 +79,7 @@
  * Unsupported supervisor features. When a supervisor feature in this mask is
  * supported in the future, move it to the supported supervisor feature mask.
  */
-#define XFEATURE_MASK_SUPERVISOR_UNSUPPORTED (XFEATURE_MASK_PT | \
-					      XFEATURE_MASK_CET_KERNEL)
+#define XFEATURE_MASK_SUPERVISOR_UNSUPPORTED (XFEATURE_MASK_PT)
 
 /* All supervisor states including supported and unsupported states. */
 #define XFEATURE_MASK_SUPERVISOR_ALL (XFEATURE_MASK_SUPERVISOR_SUPPORTED | \
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index e77cbfd18094..549cc8929407 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -56,7 +56,7 @@ static const char *xfeature_names[] =
 	"Protection Keys User registers",
 	"PASID state",
 	"Control-flow User registers",
-	"Control-flow Kernel registers (unused)",
+	"Control-flow Kernel registers (KVM only)",
 	"unknown xstate feature",
 	"unknown xstate feature",
 	"unknown xstate feature",
@@ -80,6 +80,7 @@ static unsigned short xsave_cpuid_features[] __initdata = {
 	[XFEATURE_PKRU]				= X86_FEATURE_OSPKE,
 	[XFEATURE_PASID]			= X86_FEATURE_ENQCMD,
 	[XFEATURE_CET_USER]			= X86_FEATURE_SHSTK,
+	[XFEATURE_CET_KERNEL]			= X86_FEATURE_SHSTK,
 	[XFEATURE_XTILE_CFG]			= X86_FEATURE_AMX_TILE,
 	[XFEATURE_XTILE_DATA]			= X86_FEATURE_AMX_TILE,
 	[XFEATURE_APX]				= X86_FEATURE_APX,
@@ -371,6 +372,7 @@ static __init void os_xrstor_booting(struct xregs_state *xstate)
 	 XFEATURE_MASK_BNDCSR |			\
 	 XFEATURE_MASK_PASID |			\
 	 XFEATURE_MASK_CET_USER |		\
+	 XFEATURE_MASK_CET_KERNEL |		\
 	 XFEATURE_MASK_XTILE |			\
 	 XFEATURE_MASK_APX)
 
@@ -572,6 +574,7 @@ static bool __init check_xstate_against_struct(int nr)
 	case XFEATURE_PASID:	  return XCHECK_SZ(sz, nr, struct ia32_pasid_state);
 	case XFEATURE_XTILE_CFG:  return XCHECK_SZ(sz, nr, struct xtile_cfg);
 	case XFEATURE_CET_USER:	  return XCHECK_SZ(sz, nr, struct cet_user_state);
+	case XFEATURE_CET_KERNEL: return XCHECK_SZ(sz, nr, struct cet_supervisor_state);
 	case XFEATURE_APX:        return XCHECK_SZ(sz, nr, struct apx_state);
 	case XFEATURE_XTILE_DATA: check_xtile_data_against_struct(sz); return true;
 	default:
-- 
2.47.1

[tip: x86/fpu] x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state_perm

[tip-bot2 for Sean Christopherson]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     d8414603b29f25191fcceafb72e74b67ac2e92b1
Gitweb:        https://git.kernel.org/tip/d8414603b29f25191fcceafb72e74b67ac2e92b1
Author:        Sean Christopherson <seanjc@google.com>
AuthorDate:    Tue, 06 May 2025 17:36:06 +08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 06 May 2025 11:42:04 +02:00

x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state_perm

When granting userspace or a KVM guest access to an xfeature, preserve the
entity's existing supervisor and software-defined permissions as tracked
by __state_perm, i.e. use __state_perm to track *all* permissions even
though all supported supervisor xfeatures are granted to all FPUs and
FPU_GUEST_PERM_LOCKED disallows changing permissions.

Effectively clobbering supervisor permissions results in inconsistent
behavior, as xstate_get_group_perm() will report supervisor features for
process that do NOT request access to dynamic user xfeatures, whereas any
and all supervisor features will be absent from the set of permissions for
any process that is granted access to one or more dynamic xfeatures (which
right now means AMX).

The inconsistency isn't problematic because fpu_xstate_prctl() already
strips out everything except user xfeatures:

        case ARCH_GET_XCOMP_PERM:
                /*
                 * Lockless snapshot as it can also change right after the
                 * dropping the lock.
                 */
                permitted = xstate_get_host_group_perm();
                permitted &= XFEATURE_MASK_USER_SUPPORTED;
                return put_user(permitted, uptr);

        case ARCH_GET_XCOMP_GUEST_PERM:
                permitted = xstate_get_guest_group_perm();
                permitted &= XFEATURE_MASK_USER_SUPPORTED;
                return put_user(permitted, uptr);

and similarly KVM doesn't apply the __state_perm to supervisor states
(kvm_get_filtered_xcr0() incorporates xstate_get_guest_group_perm()):

        case 0xd: {
                u64 permitted_xcr0 = kvm_get_filtered_xcr0();
                u64 permitted_xss = kvm_caps.supported_xss;

But if KVM in particular were to ever change, dropping supervisor
permissions would result in subtle bugs in KVM's reporting of supported
CPUID settings.  And the above behavior also means that having supervisor
xfeatures in __state_perm is correctly handled by all users.

Dropping supervisor permissions also creates another landmine for KVM.  If
more dynamic user xfeatures are ever added, requesting access to multiple
xfeatures in separate ARCH_REQ_XCOMP_GUEST_PERM calls will result in the
second invocation of __xstate_request_perm() computing the wrong ksize, as
as the mask passed to xstate_calculate_size() would not contain *any*
supervisor features.

Commit 781c64bfcb73 ("x86/fpu/xstate: Handle supervisor states in XSTATE
permissions") fudged around the size issue for userspace FPUs, but for
reasons unknown skipped guest FPUs.  Lack of a fix for KVM "works" only
because KVM doesn't yet support virtualizing features that have supervisor
xfeatures, i.e. as of today, KVM guest FPUs will never need the relevant
xfeatures.

Simply extending the hack-a-fix for guests would temporarily solve the
ksize issue, but wouldn't address the inconsistency issue and would leave
another lurking pitfall for KVM.  KVM support for virtualizing CET will
likely add CET_KERNEL as a guest-only xfeature, i.e. CET_KERNEL will not
be set in xfeatures_mask_supervisor() and would again be dropped when
granting access to dynamic xfeatures.

Note, the existing clobbering behavior is rather subtle.  The @permitted
parameter to __xstate_request_perm() comes from:

	permitted = xstate_get_group_perm(guest);

which is either fpu->guest_perm.__state_perm or fpu->perm.__state_perm,
where __state_perm is initialized to:

        fpu->perm.__state_perm          = fpu_kernel_cfg.default_features;

and copied to the guest side of things:

	/* Same defaults for guests */
	fpu->guest_perm = fpu->perm;

fpu_kernel_cfg.default_features contains everything except the dynamic
xfeatures, i.e. everything except XFEATURE_MASK_XTILE_DATA:

        fpu_kernel_cfg.default_features = fpu_kernel_cfg.max_features;
        fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;

When __xstate_request_perm() restricts the local "mask" variable to
compute the user state size:

	mask &= XFEATURE_MASK_USER_SUPPORTED;
	usize = xstate_calculate_size(mask, false);

it subtly overwrites the target __state_perm with "mask" containing only
user xfeatures:

	perm = guest ? &fpu->guest_perm : &fpu->perm;
	/* Pairs with the READ_ONCE() in xstate_get_group_perm() */
	WRITE_ONCE(perm->__state_perm, mask);

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: John Allen <john.allen@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mitchell Levy <levymitchell0@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Samuel Holland <samuel.holland@sifive.com>
Cc: Sohil Mehta <sohil.mehta@intel.com>
Cc: Vignesh Balasubramanian <vigbalas@amd.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Xin Li <xin3.li@intel.com>
Cc: kvm@vger.kernel.org
Link: https://lore.kernel.org/all/ZTqgzZl-reO1m01I@google.com
Link: https://lore.kernel.org/r/20250506093740.2864458-2-chao.gao@intel.com
---
 arch/x86/include/asm/fpu/types.h |  8 +++++---
 arch/x86/kernel/fpu/xstate.c     | 18 +++++++++++-------
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 97310df..e64db0e 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -416,9 +416,11 @@ struct fpu_state_perm {
 	/*
 	 * @__state_perm:
 	 *
-	 * This bitmap indicates the permission for state components, which
-	 * are available to a thread group. The permission prctl() sets the
-	 * enabled state bits in thread_group_leader()->thread.fpu.
+	 * This bitmap indicates the permission for state components
+	 * available to a thread group, including both user and supervisor
+	 * components and software-defined bits like FPU_GUEST_PERM_LOCKED.
+	 * The permission prctl() sets the enabled state bits in
+	 * thread_group_leader()->thread.fpu.
 	 *
 	 * All run time operations use the per thread information in the
 	 * currently active fpu.fpstate which contains the xfeature masks
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 8b14c9d..1c8410b 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1656,16 +1656,20 @@ static int __xstate_request_perm(u64 permitted, u64 requested, bool guest)
 	if ((permitted & requested) == requested)
 		return 0;
 
-	/* Calculate the resulting kernel state size */
+	/*
+	 * Calculate the resulting kernel state size.  Note, @permitted also
+	 * contains supervisor xfeatures even though supervisor are always
+	 * permitted for kernel and guest FPUs, and never permitted for user
+	 * FPUs.
+	 */
 	mask = permitted | requested;
-	/* Take supervisor states into account on the host */
-	if (!guest)
-		mask |= xfeatures_mask_supervisor();
 	ksize = xstate_calculate_size(mask, compacted);
 
-	/* Calculate the resulting user state size */
-	mask &= XFEATURE_MASK_USER_SUPPORTED;
-	usize = xstate_calculate_size(mask, false);
+	/*
+	 * Calculate the resulting user state size.  Take care not to clobber
+	 * the supervisor xfeatures in the new mask!
+	 */
+	usize = xstate_calculate_size(mask & XFEATURE_MASK_USER_SUPPORTED, false);
 
 	if (!guest) {
 		ret = validate_sigaltstack(usize);

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

[Sean Christopherson]

On Tue, May 06, 2025, Chao Gao wrote:
> 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.
> 
> Additionally, incorporate the initialization of indicators (is_valloc and
> is_guest) into the newly added guest-specific reset function to centralize
> the resetting of guest fpstate.
> 
> Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
> v6: Drop vcpu_fpu_config.user_* (Rick)
> v5: init is_valloc/is_guest in the guest-specific reset function (Chang)
> ---
>  arch/x86/kernel/fpu/core.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 444e517a8648..78a9c809dfad 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -211,7 +211,24 @@ void fpu_reset_from_exception_fixup(void)
>  }
>  
>  #if IS_ENABLED(CONFIG_KVM)
> -static void __fpstate_reset(struct fpstate *fpstate, u64 xfd);
> +static void __guest_fpstate_reset(struct fpstate *fpstate, u64 xfd)
> +{
> +	/*
> +	 * Initialize sizes and feature masks. Supervisor features and
> +	 * sizes may diverge between guest FPUs and host FPUs, whereas
> +	 * user features and sizes remain the same.
> +	 */
> +	fpstate->size		= guest_default_cfg.size;
> +	fpstate->xfeatures	= guest_default_cfg.features;
> +	fpstate->user_size	= fpu_user_cfg.default_size;
> +	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
> +	fpstate->xfd		= xfd;
> +
> +	/* Initialize indicators to reflect properties of the fpstate */
> +	fpstate->is_valloc	= true;
> +	fpstate->is_guest	= true;
> +}
> +

Extra newline.
>  
>  static void fpu_lock_guest_permissions(void)
>  {
> @@ -236,19 +253,18 @@ 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;
>  
>  	/* Leave xfd to 0 (the reset value defined by spec) */
> -	__fpstate_reset(fpstate, 0);
> +	__guest_fpstate_reset(fpstate, 0);

Given that there is a single caller for each of __fpstate_reset() and
__guest_fpstate_reset(), keeping the helpers does more harm than good IMO.
Passing in '0' and setting xfd in __guest_fpstate_reset() is especially pointless.

E.g.

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index d5f3af2ba758..87d6ee87ff55 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -212,25 +212,6 @@ void fpu_reset_from_exception_fixup(void)
 }
 
 #if IS_ENABLED(CONFIG_KVM)
-static void __guest_fpstate_reset(struct fpstate *fpstate, u64 xfd)
-{
-       /*
-        * Initialize sizes and feature masks. Supervisor features and
-        * sizes may diverge between guest FPUs and host FPUs, whereas
-        * user features and sizes remain the same.
-        */
-       fpstate->size           = guest_default_cfg.size;
-       fpstate->xfeatures      = guest_default_cfg.features;
-       fpstate->user_size      = fpu_user_cfg.default_size;
-       fpstate->user_xfeatures = fpu_user_cfg.default_features;
-       fpstate->xfd            = xfd;
-
-       /* Initialize indicators to reflect properties of the fpstate */
-       fpstate->is_valloc      = true;
-       fpstate->is_guest       = true;
-}
-
-
 static void fpu_lock_guest_permissions(void)
 {
        struct fpu_state_perm *fpuperm;
@@ -260,8 +241,20 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
        if (!fpstate)
                return false;
 
-       /* Leave xfd to 0 (the reset value defined by spec) */
-       __guest_fpstate_reset(fpstate, 0);
+       /* Initialize indicators to reflect properties of the fpstate */
+       fpstate->is_valloc      = true;
+       fpstate->is_guest       = true;
+
+       /*
+        * 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.
+        */
+       fpstate->size           = guest_default_cfg.size;
+       fpstate->xfeatures      = guest_default_cfg.features;
+       fpstate->user_size      = fpu_user_cfg.default_size;
+       fpstate->user_xfeatures = fpu_user_cfg.default_features;
+
        fpstate_init_user(fpstate);
 
        gfpu->fpstate           = fpstate;
@@ -550,21 +543,17 @@ void fpstate_init_user(struct fpstate *fpstate)
                fpstate_init_fstate(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            = xfd;
-}
-
 void fpstate_reset(struct fpu *fpu)
 {
        /* Set the fpstate pointer to the default fpstate */
        fpu->fpstate = &fpu->__fpstate;
-       __fpstate_reset(fpu->fpstate, init_fpstate.xfd);
+
+       /* Initialize sizes and feature masks */
+       fpu->fpstate->size              = fpu_kernel_cfg.default_size;
+       fpu->fpstate->user_size         = fpu_user_cfg.default_size;
+       fpu->fpstate->xfeatures         = fpu_kernel_cfg.default_features;
+       fpu->fpstate->user_xfeatures    = fpu_user_cfg.default_features;
+       fpu->fpstate->xfd               = init_fpstate.xfd;
 
        /* Initialize the permission related info in fpu */
        fpu->perm.__state_perm          = fpu_kernel_cfg.default_features;

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

[Chao Gao]

>>  static void fpu_lock_guest_permissions(void)
>>  {
>> @@ -236,19 +253,18 @@ 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;
>>  
>>  	/* Leave xfd to 0 (the reset value defined by spec) */
>> -	__fpstate_reset(fpstate, 0);
>> +	__guest_fpstate_reset(fpstate, 0);
>
>Given that there is a single caller for each of __fpstate_reset() and
>__guest_fpstate_reset(), keeping the helpers does more harm than good IMO.

Seems Dave strongly dislikes inlining the helpers

https://lore.kernel.org/kvm/98dda94f-b805-4e0e-871a-085eb2f6ff20@intel.com/

>Passing in '0' and setting xfd in __guest_fpstate_reset() is especially pointless.

Agreed.

Giving Dave's feedback, how about tweaking __fpstate_reset() for guest FPUs:

From 13eddc8a0ecbd9e2a59dc1caacc7dbf198277a24 Mon Sep 17 00:00:00 2001
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;
+	}
+
 	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

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

[Sean Christopherson]

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

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

[Dave Hansen]

On 5/7/25 11:01, Sean Christopherson wrote:
> 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;
> 	}

That looks nice because it clearly highlights how guests and bare-metal
fpstates are different. I like it too.

Re: [PATCH v6 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

[Edgecombe, Rick P]

On Tue, 2025-05-06 at 17:36 +0800, Chao Gao wrote:
> Currently, guest and host FPUs share the same default features. However,
> the CET supervisor xstate is the first feature that needs to be enabled
> exclusively for guest FPUs. Enabling it for host FPUs leads to a waste of
> 24 bytes in the XSAVE buffer.
> 
> To support "guest-only" features, add a new structure to hold the
> default features and sizes for guest FPUs to clearly differentiate them
> from those for host FPUs.
> 
> Note that,
> 1) for now, the default features for guest and host FPUs remain the
> same. This will change in a follow-up patch once guest permissions, default
> xfeatures, and fpstate size are all converted to use the guest defaults.
> 
> 2) only supervisor features will diverge between guest FPUs and host
> FPUs, while user features will remain the same [1][2]. So, the new
> vcpu_fpu_config struct does not include default user features and size
> for the UABI buffer.
> 
> An alternative approach is adding a guest_only_xfeatures member to
> fpu_kernel_cfg and adding two helper functions to calculate the guest
> default xfeatures and size. However, calculating these defaults at runtime
> would introduce unnecessary overhead.
> 
> Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

[PATCH v6a 5/8] x86/fpu: Initialize guest fpstate and FPU pseudo container from guest defaults

[Chao Gao]

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>
---
v6a: tweak __fpstate_reset() instead of adding a guest-specific reset
     function (Sean/Dave)
v6: Drop vcpu_fpu_config.user_* (Rick)
v5: init is_valloc/is_guest in the guest-specific reset function
(Chang)

Note: this quick revision is just intended to ensure that the feedback
has been properly addressed.

 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;
+	}
+
 	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

[PATCH v6a 6/8] x86/fpu: Remove xfd argument from __fpstate_reset()

[Chao Gao]

The initial values for fpstate::xfd differ between guest and host fpstates.
Currently, the initial values are passed as an argument to
__fpstate_reset(). But, __fpstate_reset() already assigns different default
features and sizes based on the type of fpstates (i.e., guest or host). So,
handle fpstate::xfd in a similar way to highlight the differences in the
initial xfd value between guest and host fpstates

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/all/aBuf7wiiDT0Wflhk@google.com/
---
v6a: new.

Note: this quick revision is just intended to ensure that the feedback
has been properly addressed.

 arch/x86/kernel/fpu/core.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 0d501bd25d79..a3cafed350e0 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -211,7 +211,7 @@ void fpu_reset_from_exception_fixup(void)
 }
 
 #if IS_ENABLED(CONFIG_KVM)
-static void __fpstate_reset(struct fpstate *fpstate, u64 xfd);
+static void __fpstate_reset(struct fpstate *fpstate);
 
 static void fpu_lock_guest_permissions(void)
 {
@@ -246,8 +246,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
 	fpstate->is_valloc	= true;
 	fpstate->is_guest	= true;
 
-	/* Leave xfd to 0 (the reset value defined by spec) */
-	__fpstate_reset(fpstate, 0);
+	__fpstate_reset(fpstate);
 	fpstate_init_user(fpstate);
 
 	gfpu->fpstate		= fpstate;
@@ -536,7 +535,7 @@ void fpstate_init_user(struct fpstate *fpstate)
 		fpstate_init_fstate(fpstate);
 }
 
-static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
+static void __fpstate_reset(struct fpstate *fpstate)
 {
 	/*
 	 * Initialize sizes and feature masks. Supervisor features and
@@ -546,21 +545,23 @@ static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
 	if (fpstate->is_guest) {
 		fpstate->size		= guest_default_cfg.size;
 		fpstate->xfeatures	= guest_default_cfg.features;
+		/* Leave xfd to 0 (the reset value defined by spec) */
+		fpstate->xfd		= 0;
 	} else {
 		fpstate->size		= fpu_kernel_cfg.default_size;
 		fpstate->xfeatures	= fpu_kernel_cfg.default_features;
+		fpstate->xfd		= init_fpstate.xfd;
 	}
 
 	fpstate->user_size	= fpu_user_cfg.default_size;
 	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
-	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, init_fpstate.xfd);
+	__fpstate_reset(fpu->fpstate);
 
 	/* Initialize the permission related info in fpu */
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
-- 
2.47.1

Re: [PATCH v6a 6/8] x86/fpu: Remove xfd argument from __fpstate_reset()

[Sean Christopherson]

On Fri, May 09, 2025, Chao Gao wrote:
> The initial values for fpstate::xfd differ between guest and host fpstates.
> Currently, the initial values are passed as an argument to
> __fpstate_reset(). But, __fpstate_reset() already assigns different default
> features and sizes based on the type of fpstates (i.e., guest or host). So,
> handle fpstate::xfd in a similar way to highlight the differences in the
> initial xfd value between guest and host fpstates
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Link: https://lore.kernel.org/all/aBuf7wiiDT0Wflhk@google.com/
> ---
> v6a: new.
> 
> Note: this quick revision is just intended to ensure that the feedback
> has been properly addressed.

Both patches LGTM.