Re: [PATCH 1/5] x86, fpu: introduce fpu_load_guest_fpstate()

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 26, 2025, Yao Yuan wrote:
> On Wed, Dec 24, 2025 at 01:12:45AM +0800, Paolo Bonzini wrote:
> > Create a variant of fpregs_lock_and_load() that KVM can use in its
> > vCPU entry code after preemption has been disabled.  While basing
> > it on the existing logic in vcpu_enter_guest(), ensure that
> > fpregs_assert_state_consistent() always runs and sprinkle a few
> > more assertions.
> >
> > Cc: stable@vger.kernel.org
> > Fixes: 820a6ee944e7 ("kvm: x86: Add emulation for IA32_XFD", 2022-01-14)
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  arch/x86/include/asm/fpu/api.h |  1 +
> >  arch/x86/kernel/fpu/core.c     | 17 +++++++++++++++++
> >  arch/x86/kvm/x86.c             |  8 +-------
> >  3 files changed, 19 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
> > index cd6f194a912b..0820b2621416 100644
> > --- a/arch/x86/include/asm/fpu/api.h
> > +++ b/arch/x86/include/asm/fpu/api.h
> > @@ -147,6 +147,7 @@ extern void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr);
> >  /* KVM specific functions */
> >  extern bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu);
> >  extern void fpu_free_guest_fpstate(struct fpu_guest *gfpu);
> > +extern void fpu_load_guest_fpstate(struct fpu_guest *gfpu);
> >  extern int fpu_swap_kvm_fpstate(struct fpu_guest *gfpu, bool enter_guest);
> >  extern int fpu_enable_guest_xfd_features(struct fpu_guest *guest_fpu, u64 xfeatures);
> >
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index 3ab27fb86618..a480fa8c65d5 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -878,6 +878,23 @@ void fpregs_lock_and_load(void)
> >  	fpregs_assert_state_consistent();
> >  }
> >
> > +void fpu_load_guest_fpstate(struct fpu_guest *gfpu)
> > +{
> > +#ifdef CONFIG_X86_DEBUG_FPU
> > +	struct fpu *fpu = x86_task_fpu(current);
> > +	WARN_ON_ONCE(gfpu->fpstate != fpu->fpstate);
> > +#endif
> > +
> > +	lockdep_assert_preemption_disabled();
> 
> Hi Paolo,
> 
> Do we need make sure the irq is disabled w/ lockdep ?

Yes please, e.g. see commit 2620fe268e80 ("KVM: x86: Revert "KVM: X86: Fix fpu
state crash in kvm guest"").

> The irq_fpu_usable() returns true for:
> 
> !in_nmi () && in_hardirq() and !softirq_count()
> 
> It's possible that the TIF_NEED_FPU_LOAD is set again
> w/ interrupt is enabled.