Re: [RFC PATCH v2 16/23] KVM: arm64: Enumerate SVE register indices for KVM_GET_REG_LIST

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Nov 21, 2018 at 04:09:03PM +0000, Alex Bennée wrote:
> 
> Dave Martin <Dave.Martin@arm.com> writes:
> 
> > This patch includes the SVE register IDs in the list returned by
> > KVM_GET_REG_LIST, as appropriate.
> >
> > On a non-SVE-enabled vcpu, no extra IDs are added.
> >
> > On an SVE-enabled vcpu, the appropriate number of slice IDs are
> > enumerated for each SVE register, depending on the maximum vector
> > length for the vcpu.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> >
> > Changes since RFCv1:
> >
> >  * Simplify enumerate_sve_regs() based on Andrew Jones' approach.
> >
> >  * Reg copying loops are inverted for brevity, since the order we
> >    spit out the regs in doesn't really matter.
> >
> > (I tried to keep part of my approach to avoid the duplicate logic
> > between num_sve_regs() and copy_sve_reg_indices(), but although
> > it works in principle, gcc fails to fully collapse the num_regs()
> > case... so I gave up.  The two functions need to be manually kept
> > consistent, but hopefully that's fairly straightforward.)
> > ---
> >  arch/arm64/kvm/guest.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 45 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index 320db0f..89eab68 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -323,6 +323,46 @@ static int get_timer_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  	return copy_to_user(uaddr, &val, KVM_REG_SIZE(reg->id)) ? -EFAULT : 0;
> >  }
> >
> > +static unsigned long num_sve_regs(const struct kvm_vcpu *vcpu)
> > +{
> > +	const unsigned int slices = DIV_ROUND_UP(
> > +		vcpu->arch.sve_max_vl,
> > +		KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0)));
> 
> Having seen this formulation come up several times now I wonder if there
> should be a kernel private define, KVM_SVE_ZREG/PREG_SIZE to avoid this
> clumsiness.

I agree it's a bit awkward.  Previous I spelled this "0x100", which
was terse but more sensitive to typos and other screwups that Io
liked.

> You could still use the KVM_REG_SIZE to extract it as I guess this is to
> make changes simpler if/when the SVE reg size gets bumped up.

That might be more challenging to determine at compile time.

I'm not sure how good GCC is at doing const-propagation between related
(but different) expressions, so I preferred to go for something that
is clearly compiletime constant rather than extracting it from the
register ID that came from userspace.

So, I'd prefer not to use KVM_REG_SIZE() for this, but I'm happy to add
a private #define to hide this cumbersome construct.  That would
certainly make the code more readable.

(Of course, the actual runtime cost is trivial either way, but I felt
it was easier to reason about correctness if this is really a constant.)


Sound OK?

 > 
> > +
> > +	if (!vcpu_has_sve(vcpu))
> > +		return 0;
> > +
> > +	return slices * (SVE_NUM_PREGS + SVE_NUM_ZREGS + 1 /* FFR */);
> > +}
> > +
> > +static int copy_sve_reg_indices(const struct kvm_vcpu *vcpu, u64 __user **uind)
> > +{
> > +	const unsigned int slices = DIV_ROUND_UP(
> > +		vcpu->arch.sve_max_vl,
> > +		KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0)));
> > +	unsigned int i, n;
> > +
> > +	if (!vcpu_has_sve(vcpu))
> > +		return 0;
> > +
> > +	for (i = 0; i < slices; i++) {
> > +		for (n = 0; n < SVE_NUM_ZREGS; n++) {
> > +			if (put_user(KVM_REG_ARM64_SVE_ZREG(n, i), (*uind)++))
> > +				return -EFAULT;
> > +		}
> > +
> > +		for (n = 0; n < SVE_NUM_PREGS; n++) {
> > +			if (put_user(KVM_REG_ARM64_SVE_PREG(n, i), (*uind)++))
> > +				return -EFAULT;
> > +		}
> > +
> > +		if (put_user(KVM_REG_ARM64_SVE_FFR(i), (*uind)++))
> > +			return -EFAULT;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >  /**
> >   * kvm_arm_num_regs - how many registers do we present via KVM_GET_ONE_REG
> >   *
> > @@ -333,6 +373,7 @@ unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu)
> >  	unsigned long res = 0;
> >
> >  	res += num_core_regs();
> > +	res += num_sve_regs(vcpu);
> >  	res += kvm_arm_num_sys_reg_descs(vcpu);
> >  	res += kvm_arm_get_fw_num_regs(vcpu);
> >  	res += NUM_TIMER_REGS;
> > @@ -357,6 +398,10 @@ int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
> >  		uindices++;
> >  	}
> >
> > +	ret = copy_sve_reg_indices(vcpu, &uindices);
> > +	if (ret)
> > +		return ret;
> > +
> >  	ret = kvm_arm_copy_fw_reg_indices(vcpu, uindices);
> >  	if (ret)
> >  		return ret;
> 
> Otherwise:
> 
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Thanks
---Dave