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x86/fpu: Provide infrastructure for KVM FPU cleanup

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For the upcoming AMX support it's necessary to do a proper integration with
KVM. Currently KVM allocates two FPU structs which are used for saving the user
state of the vCPU thread and restoring the guest state when entering
vcpu_run() and doing the reverse operation before leaving vcpu_run().

With the new fpstate mechanism this can be reduced to one extra buffer by
swapping the fpstate pointer in current:🧵:fpu. This makes the
upcoming support for AMX and XFD simpler because then fpstate information
(features, sizes, xfd) are always consistent and it does not require any
nasty workarounds.

Provide:

  - An allocator which initializes the state properly

  - A replacement for the existing FPU swap mechanim

Aside of the reduced memory footprint, this also makes state switching
more efficient when TIF_FPU_NEED_LOAD is set. It does not require a
memcpy as the state is already correct in the to be swapped out fpstate.

The existing interfaces will be removed once KVM is converted over.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211022185312.954684740@linutronix.de
This commit is contained in:
Thomas Gleixner 2021-10-22 20:55:51 +02:00 • committed by Borislav Petkov
parent 75c52dad5e
commit 69f6ed1d14
2 changed files with 92 additions and 6 deletions
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13
arch/x86/include/asm/fpu/api.h
View file

@ -135,9 +135,22 @@ extern void fpu_init_fpstate_user(struct fpu *fpu);
extern void fpstate_clear_xstate_component(struct fpstate *fps, unsigned int xfeature);
/* KVM specific functions */
extern bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu);
extern void fpu_free_guest_fpstate(struct fpu_guest *gfpu);
extern int fpu_swap_kvm_fpstate(struct fpu_guest *gfpu, bool enter_guest);
extern void fpu_swap_kvm_fpu(struct fpu *save, struct fpu *rstor, u64 restore_mask);
extern int fpu_copy_kvm_uabi_to_fpstate(struct fpu *fpu, const void *buf, u64 xcr0, u32 *pkru);
extern void fpu_copy_fpstate_to_kvm_uabi(struct fpu *fpu, void *buf, unsigned int size, u32 pkru);
static inline void fpstate_set_confidential(struct fpu_guest *gfpu)
{
gfpu->fpstate->is_confidential = true;
}
static inline bool fpstate_is_confidential(struct fpu_guest *gfpu)
{
return gfpu->fpstate->is_confidential;
}
#endif /* _ASM_X86_FPU_API_H */

85
arch/x86/kernel/fpu/core.c
View file

@ -176,6 +176,75 @@ void fpu_reset_from_exception_fixup(void)
}
#if IS_ENABLED(CONFIG_KVM)
static void __fpstate_reset(struct fpstate *fpstate);
bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
{
struct fpstate *fpstate;
unsigned int size;
size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
fpstate = vzalloc(size);
if (!fpstate)
return false;
__fpstate_reset(fpstate);
fpstate_init_user(fpstate);
fpstate->is_valloc = true;
fpstate->is_guest = true;
gfpu->fpstate = fpstate;
return true;
}
EXPORT_SYMBOL_GPL(fpu_alloc_guest_fpstate);
void fpu_free_guest_fpstate(struct fpu_guest *gfpu)
{
struct fpstate *fps = gfpu->fpstate;
if (!fps)
return;
if (WARN_ON_ONCE(!fps->is_valloc || !fps->is_guest || fps->in_use))
return;
gfpu->fpstate = NULL;
vfree(fps);
}
EXPORT_SYMBOL_GPL(fpu_free_guest_fpstate);
int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)
{
struct fpstate *guest_fps = guest_fpu->fpstate;
struct fpu *fpu = &current->thread.fpu;
struct fpstate *cur_fps = fpu->fpstate;
fpregs_lock();
if (!cur_fps->is_confidential && !test_thread_flag(TIF_NEED_FPU_LOAD))
save_fpregs_to_fpstate(fpu);
/* Swap fpstate */
if (enter_guest) {
fpu->__task_fpstate = cur_fps;
fpu->fpstate = guest_fps;
guest_fps->in_use = true;
} else {
guest_fps->in_use = false;
fpu->fpstate = fpu->__task_fpstate;
fpu->__task_fpstate = NULL;
}
cur_fps = fpu->fpstate;
if (!cur_fps->is_confidential)
restore_fpregs_from_fpstate(cur_fps, XFEATURE_MASK_FPSTATE);
fpregs_mark_activate();
fpregs_unlock();
return 0;
}
EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpstate);
void fpu_swap_kvm_fpu(struct fpu *save, struct fpu *rstor, u64 restore_mask)
{
fpregs_lock();
@ -352,16 +421,20 @@ void fpstate_init_user(struct fpstate *fpstate)
fpstate_init_fstate(fpstate);
}
static void __fpstate_reset(struct fpstate *fpstate)
{
/* 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;
}
void fpstate_reset(struct fpu *fpu)
{
/* Set the fpstate pointer to the default fpstate */
fpu->fpstate = &fpu->__fpstate;
/* 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;
__fpstate_reset(fpu->fpstate);
}
#if IS_ENABLED(CONFIG_KVM)