Subject: [PATCH v6 1/6] arm/arm64: KVM: Introduce armv7 fp/simd vcpu fields and helpers

From: [email-masked] (Christoffer Dall)
Date: Tue, 12 Jan 2016 15:12:07 +0100

On Mon, Jan 11, 2016 at 03:39:21PM -0800, Mario Smarduch wrote:
> 
> 
> On 1/10/2016 8:32 AM, Christoffer Dall wrote:
> > Hi Mario,
> > 
> > I spotted one more potential issue...
> > 
> > On Sat, Dec 26, 2015 at 01:54:55PM -0800, Mario Smarduch wrote:
> >> Add helper functions to enable access to fp/smid on guest entry and save host
> >> fpexc on vcpu put, check if fp/simd registers are dirty and add new vcpu
> >> fields.
> >>
> >> Signed-off-by: Mario Smarduch <[email-masked]>
> >> ---
> >>  arch/arm/include/asm/kvm_emulate.h   | 42 ++++++++++++++++++++++++++++++++++++
> >>  arch/arm/include/asm/kvm_host.h      |  6 ++++++
> >>  arch/arm64/include/asm/kvm_emulate.h |  8 +++++++
> >>  3 files changed, 56 insertions(+)
> >>
> >> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> >> index 3095df0..d4d9da1 100644
> >> --- a/arch/arm/include/asm/kvm_emulate.h
> >> +++ b/arch/arm/include/asm/kvm_emulate.h
> >> @@ -24,6 +24,8 @@
> >>  #include <asm/kvm_mmio.h>
> >>  #include <asm/kvm_arm.h>
> >>  #include <asm/cputype.h>
> >> +#include <asm/vfp.h>
> >> +#include "../vfp/vfpinstr.h"
> >>  
> >>  unsigned long *vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num);
> >>  unsigned long *vcpu_spsr(struct kvm_vcpu *vcpu);
> >> @@ -255,4 +257,44 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
> >>  	}
> >>  }
> >>  
> >> +#ifdef CONFIG_VFPv3
> >> +/* Called from vcpu_load - save fpexc and enable guest access to fp/simd unit */
> >> +static inline void vcpu_trap_vfp_enable(struct kvm_vcpu *vcpu)
> >> +{
> >> +	u32 fpexc;
> >> +
> >> +	/* Save host fpexc, and enable guest access to fp unit */
> >> +	fpexc = fmrx(FPEXC);
> >> +	vcpu->arch.host_fpexc = fpexc;
> >> +	fpexc |= FPEXC_EN;
> >> +	fmxr(FPEXC, fpexc);
> >> +
> >> +	/* Configure HCPTR to trap on tracing and fp/simd access */
> >> +	vcpu->arch.hcptr = HCPTR_TTA | HCPTR_TCP(10)  | HCPTR_TCP(11);
> >> +}
> >> +
> >> +/* Called from vcpu_put - restore host fpexc */
> >> +static inline void vcpu_restore_host_fpexc(struct kvm_vcpu *vcpu)
> >> +{
> >> +	fmxr(FPEXC, vcpu->arch.host_fpexc);
> >> +}
> >> +
> >> +/* If trap bits are reset then fp/simd registers are dirty */
> >> +static inline bool vcpu_vfp_isdirty(struct kvm_vcpu *vcpu)
> >> +{
> >> +	return !(vcpu->arch.hcptr & (HCPTR_TCP(10) | HCPTR_TCP(11)));
> >> +}
> >> +#else
> >> +static inline void vcpu_trap_vfp_enable(struct kvm_vcpu *vcpu)
> >> +{
> >> +	vcpu->arch.hcptr = HCPTR_TTA;
> > 
> > Is it correct not to trap VFP registers when the host kernel does not
> > have CONFIG_VFPv3?  I think this is a change in functionality compared
> > to the current kernels is it not?
> 
> With CPU_V7 VFPv3 gets selected, without it fp should be emulated,
> with exceptions taken in guest kernel. I don't see a reason why
> fp hcptr access should be enabled in that case.
> 

If you have to guests with CONFIG_VFPV3 but your host doesn't have
CONFIG_VFPV3, you will never context-switch the VFP registers between
the two VMs, and mayhem will ensue.

Unless I'm missing something very obvious?

-Christoffer