Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

Paolo Bonzini
2021-10-12T17:22:24Z

On 12/10/21 02:00, Thomas Gleixner wrote:
> Swapping the host/guest FPU is directly fiddling with FPU internals which
> requires 5 exports. The upcoming support of dymanically enabled states
> would even need more.
>
> Implement a swap function in the FPU core code and export that instead.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: kvm@vger.kernel.org
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/include/asm/fpu/api.h | 8 +++++
> arch/x86/include/asm/fpu/internal.h | 15 +---------
> arch/x86/kernel/fpu/core.c | 30 ++++++++++++++++++---
> arch/x86/kernel/fpu/init.c | 1
> arch/x86/kernel/fpu/xstate.c | 1
> arch/x86/kvm/x86.c | 51 +++++++-----------------------------
> arch/x86/mm/extable.c | 2 -
> 7 files changed, 48 insertions(+), 60 deletions(-)
>
> --- a/arch/x86/include/asm/fpu/api.h
> +++ b/arch/x86/include/asm/fpu/api.h
> @@ -12,6 +12,8 @@
> #define _ASM_X86_FPU_API_H
> #include <linux/bottom_half.h>
>
> +#include <asm/fpu/types.h>
> +
> /*
> * Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It
> * disables preemption so be careful if you intend to use it for long periods
> @@ -108,4 +110,10 @@ extern int cpu_has_xfeatures(u64 xfeatur
>
> static inline void update_pasid(void) { }
>
> +/* FPSTATE related functions which are exported to KVM */
> +extern void fpu_init_fpstate_user(struct fpu *fpu);
> +
> +/* KVM specific functions */
> +extern void fpu_swap_kvm_fpu(struct fpu *save, struct fpu *rstor, u64 restore_mask);
> +
> #endif /* _ASM_X86_FPU_API_H */
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -74,14 +74,8 @@ static __always_inline __pure bool use_f
> return static_cpu_has(X86_FEATURE_FXSR);
> }
>
> -/*
> - * fpstate handling functions:
> - */
> -
> extern union fpregs_state init_fpstate;
> -
> extern void fpstate_init_user(union fpregs_state *state);
> -extern void fpu_init_fpstate_user(struct fpu *fpu);
>
> #ifdef CONFIG_MATH_EMULATION
> extern void fpstate_init_soft(struct swregs_state *soft);
> @@ -381,12 +375,7 @@ static inline int os_xrstor_safe(struct
> return err;
> }
>
> -extern void __restore_fpregs_from_fpstate(union fpregs_state *fpstate, u64 mask);
> -
> -static inline void restore_fpregs_from_fpstate(union fpregs_state *fpstate)
> -{
> - __restore_fpregs_from_fpstate(fpstate, xfeatures_mask_fpstate());
> -}
> +extern void restore_fpregs_from_fpstate(union fpregs_state *fpstate, u64 mask);
>
> extern bool copy_fpstate_to_sigframe(void __user *buf, void __user *fp, int size);
>
> @@ -467,7 +456,7 @@ static inline void fpregs_restore_userre
> */
> mask = xfeatures_mask_restore_user() |
> xfeatures_mask_supervisor();
> - __restore_fpregs_from_fpstate(&fpu->state, mask);
> + restore_fpregs_from_fpstate(&fpu->state, mask);
>
> fpregs_activate(fpu);
> fpu->last_cpu = cpu;
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -124,9 +124,8 @@ void save_fpregs_to_fpstate(struct fpu *
> asm volatile("fnsave %[fp]; fwait" : [fp] "=m" (fpu->state.fsave));
> frstor(&fpu->state.fsave);
> }
> -EXPORT_SYMBOL(save_fpregs_to_fpstate);
>
> -void __restore_fpregs_from_fpstate(union fpregs_state *fpstate, u64 mask)
> +void restore_fpregs_from_fpstate(union fpregs_state *fpstate, u64 mask)
> {
> /*
> * AMD K7/K8 and later CPUs up to Zen don't save/restore
> @@ -151,7 +150,31 @@ void __restore_fpregs_from_fpstate(union
> frstor(&fpstate->fsave);
> }
> }
> -EXPORT_SYMBOL_GPL(__restore_fpregs_from_fpstate);
> +
> +#if IS_ENABLED(CONFIG_KVM)
> +void fpu_swap_kvm_fpu(struct fpu *save, struct fpu *rstor, u64 restore_mask)
> +{
> + fpregs_lock();
> +
> + if (save) {
> + if (test_thread_flag(TIF_NEED_FPU_LOAD)) {
> + memcpy(&save->state, &current->thread.fpu.state,
> + fpu_kernel_xstate_size);
> + } else {
> + save_fpregs_to_fpstate(save);
> + }
> + }
> +
> + if (rstor) {
> + restore_mask &= xfeatures_mask_fpstate();
> + restore_fpregs_from_fpstate(&rstor->state, restore_mask);
> + }
> +
> + fpregs_mark_activate();
> + fpregs_unlock();
> +}
> +EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpu);
> +#endif
>
> void kernel_fpu_begin_mask(unsigned int kfpu_mask)
> {
> @@ -459,7 +482,6 @@ void fpregs_mark_activate(void)
> fpu->last_cpu = smp_processor_id();
> clear_thread_flag(TIF_NEED_FPU_LOAD);
> }
> -EXPORT_SYMBOL_GPL(fpregs_mark_activate);
>
> /*
> * x87 math exception handling:
> --- a/arch/x86/kernel/fpu/init.c
> +++ b/arch/x86/kernel/fpu/init.c
> @@ -136,7 +136,6 @@ static void __init fpu__init_system_gene
> * components into a single, continuous memory block:
> */
> unsigned int fpu_kernel_xstate_size __ro_after_init;
> -EXPORT_SYMBOL_GPL(fpu_kernel_xstate_size);
>
> /* Get alignment of the TYPE. */
> #define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test)
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -65,7 +65,6 @@ static short xsave_cpuid_features[] __in
> * XSAVE buffer, both supervisor and user xstates.
> */
> u64 xfeatures_mask_all __ro_after_init;
> -EXPORT_SYMBOL_GPL(xfeatures_mask_all);
>
> static unsigned int xstate_offsets[XFEATURE_MAX] __ro_after_init =
> { [ 0 ... XFEATURE_MAX - 1] = -1};
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -68,7 +68,9 @@
> #include <asm/mce.h>
> #include <asm/pkru.h>
> #include <linux/kernel_stat.h>
> -#include <asm/fpu/internal.h> /* Ugh! */
> +#include <asm/fpu/api.h>
> +#include <asm/fpu/xcr.h>
> +#include <asm/fpu/xstate.h>
> #include <asm/pvclock.h>
> #include <asm/div64.h>
> #include <asm/irq_remapping.h>
> @@ -9899,58 +9901,27 @@ static int complete_emulated_mmio(struct
> return 0;
> }
>
> -static void kvm_save_current_fpu(struct fpu *fpu)
> -{
> - /*
> - * If the target FPU state is not resident in the CPU registers, just
> - * memcpy() from current, else save CPU state directly to the target.
> - */
> - if (test_thread_flag(TIF_NEED_FPU_LOAD))
> - memcpy(&fpu->state, &current->thread.fpu.state,
> - fpu_kernel_xstate_size);
> - else
> - save_fpregs_to_fpstate(fpu);
> -}
> -
> /* Swap (qemu) user FPU context for the guest FPU context. */
> static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - fpregs_lock();
> -
> - kvm_save_current_fpu(vcpu->arch.user_fpu);
> -
> /*
> - * Guests with protected state can't have it set by the hypervisor,
> - * so skip trying to set it.
> + * Guest with protected state have guest_fpu == NULL which makes
> + * the swap only safe the host state. Exclude PKRU from restore as
> + * it is restored separately in kvm_x86_ops.run().
> */
> - if (vcpu->arch.guest_fpu)
> - /* PKRU is separately restored in kvm_x86_ops.run. */
> - __restore_fpregs_from_fpstate(&vcpu->arch.guest_fpu->state,
> - ~XFEATURE_MASK_PKRU);
> -
> - fpregs_mark_activate();
> - fpregs_unlock();
> -
> + fpu_swap_kvm_fpu(vcpu->arch.user_fpu, vcpu->arch.guest_fpu,
> + ~XFEATURE_MASK_PKRU);
> trace_kvm_fpu(1);
> }
>
> /* When vcpu_run ends, restore user space FPU context. */
> static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - fpregs_lock();
> -
> /*
> - * Guests with protected state can't have it read by the hypervisor,
> - * so skip trying to save it.
> + * Guest with protected state have guest_fpu == NULL which makes
> + * swap only restore the host state.
> */
> - if (vcpu->arch.guest_fpu)
> - kvm_save_current_fpu(vcpu->arch.guest_fpu);
> -
> - restore_fpregs_from_fpstate(&vcpu->arch.user_fpu->state);
> -
> - fpregs_mark_activate();
> - fpregs_unlock();
> -
> + fpu_swap_kvm_fpu(vcpu->arch.guest_fpu, vcpu->arch.user_fpu, ~0ULL);
> ++vcpu->stat.fpu_reload;
> trace_kvm_fpu(0);
> }
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -47,7 +47,7 @@ static bool ex_handler_fprestore(const s
> WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
> (void *)instruction_pointer(regs));
>
> - __restore_fpregs_from_fpstate(&init_fpstate, xfeatures_mask_fpstate());
> + restore_fpregs_from_fpstate(&init_fpstate, xfeatures_mask_fpstate());
> return true;
> }
>
>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>