Subject: [PATCH 08/37] KVM: arm64: Defer restoring host VFP state to vcpu_put

From: [email-masked] (Christoffer Dall)
Date: Sun, 26 Nov 2017 17:17:16 +0100

Hi Yury,

On Sat, Nov 25, 2017 at 10:52:21AM +0300, Yury Norov wrote:
> 
> On Thu, Oct 12, 2017 at 12:41:12PM +0200, Christoffer Dall wrote:
> > Avoid saving the guest VFP registers and restoring the host VFP
> > registers on every exit from the VM.  Only when we're about to run
> > userspace or other threads in the kernel do we really have to switch the
> > state back to the host state.
> > 
> > We still initially configure the VFP registers to trap when entering the
> > VM, but the difference is that we now leave the guest state in the
> > hardware registers while running the VM.
> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  arch/arm64/include/asm/kvm_emulate.h |  5 ++++
> >  arch/arm64/include/asm/kvm_host.h    |  3 +++
> >  arch/arm64/kernel/asm-offsets.c      |  1 +
> >  arch/arm64/kvm/hyp/entry.S           |  3 +++
> >  arch/arm64/kvm/hyp/switch.c          | 47 +++++++++++-------------------------
> >  arch/arm64/kvm/hyp/sysreg-sr.c       | 21 +++++++++++++---
> >  6 files changed, 44 insertions(+), 36 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 1fbfe96..630dd60 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -56,6 +56,11 @@ static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)
> >  	return (unsigned long *)&vcpu->arch.hcr_el2;
> >  }
> >  
> > +static inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
> > +{
> > +	return (!(vcpu->arch.hcr_el2 & HCR_RW));
> > +}
> > +
> >  static inline unsigned long *vcpu_pc(const struct kvm_vcpu *vcpu)
> >  {
> >  	return (unsigned long *)&vcpu_gp_regs(vcpu)->regs.pc;
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 7d3bfa7..5e09eb9 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -210,6 +210,9 @@ struct kvm_vcpu_arch {
> >  	/* Guest debug state */
> >  	u64 debug_flags;
> >  
> > +	/* 1 if the guest VFP state is loaded into the hardware */
> > +	u64 guest_vfp_loaded;
> 
> May it be just u8/bool?
> 
This particular field is accessed from assembly code, and I'm not sure
what guarantees the compiler makes in terms of how a u8/bool is
allocated with respect to padding and alignment, and I think that's why
we've been using u64 fields in the past.

I don't actually remember the details, but I'd rather err on the side of
caution than trying to save a few bytes.  However, if someone can
convince me there's a completely safe way to do this, then I'm happy to
change it.

Thanks,
-Christoffer