linux-kernel - Re: [PATCH 1/4] KVM: x86: Handle TIF_NEED_FPU_LOAD in kvm_{load,put}_guest

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Date:   Fri, 17 Jan 2020 10:43:34 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Derek Yerger <derek@....llc>,
        kernel@...dan.com, Thomas Lambertz <mail@...maslambertz.de>,
        Rik van Riel <riel@...riel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/4] KVM: x86: Handle TIF_NEED_FPU_LOAD in
 kvm_{load,put}_guest_fpu()

On Fri, Jan 17, 2020 at 10:31:53AM -0800, Dave Hansen wrote:
> On 1/16/20 10:26 PM, Sean Christopherson wrote:
> > Handle TIF_NEED_FPU_LOAD similar to how fpu__copy() handles the flag
> > when duplicating FPU state to a new task struct.  TIF_NEED_FPU_LOAD can
> > be set any time control is transferred out of KVM, be it voluntarily,
> > e.g. if I/O is triggered during a KVM call to get_user_pages, or
> > involuntarily, e.g. if softirq runs after an IRQ occurs.  Therefore,
> > KVM must account for TIF_NEED_FPU_LOAD whenever it is (potentially)
> > accessing CPU FPU state.
> > 
> > Fixes: 5f409e20b7945 ("x86/fpu: Defer FPU state load until return to userspace")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > ---
> >  arch/x86/kvm/x86.c | 27 ++++++++++++++++++++++++---
> >  1 file changed, 24 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cf917139de6b..0c7211491f98 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8476,8 +8476,20 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> >  {
> >  	fpregs_lock();
> >  
> > -	copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
> > -	/* PKRU is separately restored in kvm_x86_ops->run.  */
> > +	/*
> > +	 * If userspace's FPU state is not resident in the CPU registers, just
> > +	 * memcpy() from current, else save CPU state directly to user_fpu.
> > +	 */
> > +	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > +		memcpy(&vcpu->arch.user_fpu->state, &current->thread.fpu.state,
> > +		       fpu_kernel_xstate_size);
> > +	else
> > +		copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
> > +
> > +	/*
> > +	 * Load guest's FPU state to the CPU registers.  PKRU is separately
> > +	 * loaded in kvm_x86_ops->run.
> > +	 */
> >  	__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
> >  				~XFEATURE_MASK_PKRU);
> 
> Nit: it took me a minute to realize that there is both:
> 
> 	vcpu->arch.user_fpu
> and
> 	vcpu->arch.guest_fpu
> 
> It might help readability to have local variables for those, or at least
> a comment to help differentiate the two.

Or even better, add a helper to wrap the logic instead of copy+paste, e.g.:

static void kvm_save_current_fpu(struct fpu *fpu)
{
	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		memcpy(&fpu->state, &current->thread.fpu.state,
		       fpu_kernel_xstate_size);
	else
		copy_fpregs_to_fpstate(fpu);
}

> 
> 
> > @@ -8492,7 +8504,16 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> >  {
> >  	fpregs_lock();
> >  
> > -	copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
> > +	/*
> > +	 * If guest's FPU state is not resident in the CPU registers, just
> > +	 * memcpy() from current, else save CPU state directly to guest_fpu.
> > +	 */
> > +	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > +		memcpy(&vcpu->arch.guest_fpu->state, &current->thread.fpu.state,
> > +		       fpu_kernel_xstate_size);
> > +	else
> > +		copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
> > +
> >  	copy_kernel_to_fpregs(&vcpu->arch.user_fpu->state);
> >  
> >  	fpregs_mark_activate();
> 
> This also makes me wonder if we want to have copy_fpregs_to_fpstate()
> check for TIF_NEED_FPU_LOAD and complain if it's set.