linux-kernel - Re: [patch 14/31] x86/fpu: Replace KVMs homebrewn FPU copy from user

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Date:   Wed, 13 Oct 2021 14:57:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch 14/31] x86/fpu: Replace KVMs homebrewn FPU copy from user

On Tue, Oct 12, 2021, Borislav Petkov wrote:
> On Tue, Oct 12, 2021 at 02:00:19AM +0200, Thomas Gleixner wrote:
> > --- a/arch/x86/include/asm/fpu/api.h
> > +++ b/arch/x86/include/asm/fpu/api.h
> > @@ -116,4 +116,7 @@ extern void fpu_init_fpstate_user(struct
> >  /* KVM specific functions */
> >  extern void fpu_swap_kvm_fpu(struct fpu *save, struct fpu *rstor, u64 restore_mask);
> >  
> > +struct kvm_vcpu;
> > +extern int fpu_copy_kvm_uabi_to_vcpu(struct fpu *fpu, const void *buf, u64 xcr0, u32 *pkru);
> > +
> >  #endif /* _ASM_X86_FPU_API_H */
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -174,7 +174,43 @@ void fpu_swap_kvm_fpu(struct fpu *save,
> >  	fpregs_unlock();
> >  }
> >  EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpu);
> > -#endif
> > +
> > +int fpu_copy_kvm_uabi_to_vcpu(struct fpu *fpu, const void *buf, u64 xcr0,
> > +			      u32 *vpkru)
> 
> Right, except that there's no @vcpu in the args of that function. I
> guess you could call it
> 
> fpu_copy_kvm_uabi_to_buf()
> 
> and that @buf can be
> 
> vcpu->arch.guest_fpu

But the existing @buf is the userspace pointer, which semantically makes sense
because the userspace pointer is the "buffer" and the destination @fpu (and @prku)
is vCPU state, not a buffer.

That said, I also struggled with the lack of @vcpu.  What about prepending vcpu_
to fpu and to pkru?  E.g.

  int fpu_copy_kvm_uabi_to_vcpu(struct fpu *vcpu_fpu, const void *buf, u64 xcr0,
  				u32 *vcpu_pkru)