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

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Date:   Wed, 13 Oct 2021 17:16:07 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <seanjc@...gle.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     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 Wed, Oct 13 2021 at 14:57, Sean Christopherson wrote:
> 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)

I've renamed them to:

     fpu_copy_kvm_uabi_to_fpstate()
     fpu_copy_fpstate_to_kvm_uabi()

See
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git/log/?h=x86/fpu-1

Thanks,

        tglx