[PATCH 06/19] x86/fpu: Add reallocation mechanims for KVM

Yang Zhong
2021-12-07T15:09:33Z

From: Thomas Gleixner <tglx@linutronix.de>

Extend fpstate reallocation mechanism to cover guest fpu. Unlike native
tasks which have reallocation triggered from #NM handler, guest fpstate
reallocation is requested by KVM when detecting the guest intention
on using a dynamically-enabled XSAVE feature.

Since KVM currently swaps host/guest fpstate when exiting to userspace
VMM (see fpu_swap_kvm_fpstate()), deal with fpstate reallocation also
at this point.

The implication - KVM must break vcpu_run() loop to exit to userspace
VMM instead of immediately returning back to the guest when fpstate
requires reallocation. In this case KVM should set
guest_fpu::realloc_request to mark those features in related VM exit
handlers.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
---
arch/x86/kernel/fpu/core.c | 26 +++++++++++++++++++---
arch/x86/kernel/fpu/xstate.c | 43 ++++++++++++++++++++++++++++++------
arch/x86/kernel/fpu/xstate.h | 2 ++
3 files changed, 61 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index fae44fa27cdb..7a0436a0cb2c 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -261,11 +261,31 @@ void fpu_free_guest_fpstate(struct fpu_guest *gfpu)
}
EXPORT_SYMBOL_GPL(fpu_free_guest_fpstate);

+static int fpu_guest_realloc_fpstate(struct fpu_guest *guest_fpu,
+ bool enter_guest)
+{
+ /*
+ * Reallocation requests can only be handled when
+ * switching from guest to host mode.
+ */
+ if (WARN_ON_ONCE(enter_guest || !IS_ENABLED(CONFIG_X86_64))) {
+ guest_fpu->realloc_request = 0;
+ return -EUNATCH;
+ }
+ return xfd_enable_guest_features(guest_fpu);
+}
+
int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)
{
- struct fpstate *guest_fps = guest_fpu->fpstate;
+ struct fpstate *guest_fps, *cur_fps;
struct fpu *fpu = &current->thread.fpu;
- struct fpstate *cur_fps = fpu->fpstate;
+ int ret = 0;
+
+ if (unlikely(guest_fpu->realloc_request))
+ ret = fpu_guest_realloc_fpstate(guest_fpu, enter_guest);
+
+ guest_fps = guest_fpu->fpstate;
+ cur_fps = fpu->fpstate;

fpregs_lock();
if (!cur_fps->is_confidential && !test_thread_flag(TIF_NEED_FPU_LOAD))
@@ -298,7 +318,7 @@ int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)

fpregs_mark_activate();
fpregs_unlock();
- return 0;
+ return ret;
}
EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpstate);

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 9856d579aa6e..fe3d8ed3db0e 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1529,6 +1529,7 @@ static struct fpstate *fpu_install_fpstate(struct fpu *fpu,
* of that task
* @ksize: The required size for the kernel buffer
* @usize: The required size for user space buffers
+ * @guest_fpu: Pointer to a guest FPU container. NULL for host allocations
*
* Note vs. vmalloc(): If the task with a vzalloc()-allocated buffer
* terminates quickly, vfree()-induced IPIs may be a concern, but tasks
@@ -1537,7 +1538,7 @@ static struct fpstate *fpu_install_fpstate(struct fpu *fpu,
* Returns: 0 on success, -ENOMEM on allocation error.
*/
static int fpstate_realloc(u64 xfeatures, unsigned int ksize,
- unsigned int usize)
+ unsigned int usize, struct fpu_guest *guest_fpu)
{
struct fpu *fpu = &current->thread.fpu;
struct fpstate *curfps, *newfps = NULL;
@@ -1553,6 +1554,12 @@ static int fpstate_realloc(u64 xfeatures, unsigned int ksize,
newfps->user_size = usize;
newfps->is_valloc = true;

+ if (guest_fpu) {
+ newfps->is_guest = true;
+ newfps->is_confidential = curfps->is_confidential;
+ guest_fpu->user_xfeatures |= xfeatures;
+ }
+
fpregs_lock();
/*
* Ensure that the current state is in the registers before
@@ -1566,12 +1573,14 @@ static int fpstate_realloc(u64 xfeatures, unsigned int ksize,
newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;
newfps->xfd = curfps->xfd & ~xfeatures;

+ if (guest_fpu)
+ guest_fpu->fpstate = newfps;
+
curfps = fpu_install_fpstate(fpu, newfps);

/* Do the final updates within the locked region */
xstate_init_xcomp_bv(&newfps->regs.xsave, newfps->xfeatures);
xfd_update_state(newfps);
-
fpregs_unlock();

vfree(curfps);
@@ -1682,9 +1691,10 @@ static int xstate_request_perm(unsigned long idx, bool guest)
return ret;
}

-int xfd_enable_feature(u64 xfd_err)
+static int __xfd_enable_feature(u64 xfd_err, struct fpu_guest *guest_fpu)
{
u64 xfd_event = xfd_err & XFEATURE_MASK_USER_DYNAMIC;
+ struct fpu_state_perm *perm;
unsigned int ksize, usize;
struct fpu *fpu;

@@ -1697,14 +1707,16 @@ int xfd_enable_feature(u64 xfd_err)
spin_lock_irq(&current->sighand->siglock);

/* If not permitted let it die */
- if ((xstate_get_host_group_perm() & xfd_event) != xfd_event) {
+ if ((xstate_get_group_perm(!!guest_fpu) & xfd_event) != xfd_event) {
spin_unlock_irq(&current->sighand->siglock);
return -EPERM;
}

fpu = &current->group_leader->thread.fpu;
- ksize = fpu->perm.__state_size;
- usize = fpu->perm.__user_state_size;
+ perm = guest_fpu ? &fpu->guest_perm : &fpu->perm;
+ ksize = perm->__state_size;
+ usize = perm->__user_state_size;
+
/*
* The feature is permitted. State size is sufficient. Dropping
* the lock is safe here even if more features are added from
@@ -1717,10 +1729,27 @@ int xfd_enable_feature(u64 xfd_err)
* Try to allocate a new fpstate. If that fails there is no way
* out.
*/
- if (fpstate_realloc(xfd_event, ksize, usize))
+ if (fpstate_realloc(xfd_event, ksize, usize, guest_fpu))
return -EFAULT;
return 0;
}
+
+int xfd_enable_feature(u64 xfd_err)
+{
+ return __xfd_enable_feature(xfd_err, NULL);
+}
+
+int xfd_enable_guest_features(struct fpu_guest *guest_fpu)
+{
+ u64 xfd_err = guest_fpu->realloc_request & XFEATURE_MASK_USER_SUPPORTED;
+
+ guest_fpu->realloc_request = 0;
+
+ if (!xfd_err)
+ return 0;
+ return __xfd_enable_feature(xfd_err, guest_fpu);
+}
+
#else /* CONFIG_X86_64 */
static inline int xstate_request_perm(unsigned long idx, bool guest)
{
diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 98a472775c97..3254e2b5f17f 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -55,6 +55,8 @@ extern void fpu__init_system_xstate(unsigned int legacy_size);

extern void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr);

+extern int xfd_enable_guest_features(struct fpu_guest *guest_fpu);
+
static inline u64 xfeatures_mask_supervisor(void)
{
return fpu_kernel_cfg.max_features & XFEATURE_MASK_SUPERVISOR_SUPPORTED;