Re: [PATCH 09/14] KVM: arm64/sve: Simplify KVM_REG_ARM64_SVE_VLS array sizing

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

On Mon, Apr 15, 2019 at 05:20:37PM +0200, Andrew Jones wrote:
> On Fri, Apr 12, 2019 at 05:28:13PM +0100, Dave Martin wrote:
> > A complicated DIV_ROUND_UP() expression is currently written out
> > explicitly in multiple places in order to specify the size of the
> > bitmap exchanged with userspace to represent the value of the
> > KVM_REG_ARM64_SVE_VLS pseudo-register.
> > 
> > To make this more readable, this patch replaces these with a single
> > define.
> > 
> > Since the number of words in a bitmap is just the index of the last
> > word used + 1, this patch expresses the bound that way instead.
> > This should make it clearer what is being expressed.
> > 
> > Since use of DIV_ROUND_UP() was the only reason for including
> > <linux/kernel.h> in guest.c, this patch removes that #include.
> > 
> > No functional change.
> > 
> > Suggested-by: Andrew Jones <drjones@redhat.com>
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> >  arch/arm64/kvm/guest.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index 73044e3..f025a2f 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -23,7 +23,6 @@
> >  #include <linux/errno.h>
> >  #include <linux/err.h>
> >  #include <linux/nospec.h>
> > -#include <linux/kernel.h>
> >  #include <linux/kvm_host.h>
> >  #include <linux/module.h>
> >  #include <linux/stddef.h>
> > @@ -209,8 +208,10 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  #define vq_word(vq) (((vq) - SVE_VQ_MIN) / 64)
> >  #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64)
> >  
> > +#define SVE_VLS_WORDS (vq_word(SVE_VQ_MAX) + 1)
> > +
> >  static bool vq_present(
> > -	const u64 (*const vqs)[DIV_ROUND_UP(SVE_VQ_MAX - SVE_VQ_MIN + 1, 64)],
> > +	const u64 (*const vqs)[SVE_VLS_WORDS],
> >  	unsigned int vq)
> >  {
> >  	return (*vqs)[vq_word(vq)] & vq_mask(vq);
> > @@ -219,7 +220,7 @@ static bool vq_present(
> >  static int get_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  {
> >  	unsigned int max_vq, vq;
> > -	u64 vqs[DIV_ROUND_UP(SVE_VQ_MAX - SVE_VQ_MIN + 1, 64)];
> > +	u64 vqs[SVE_VLS_WORDS];
> >  
> >  	if (!vcpu_has_sve(vcpu))
> >  		return -ENOENT;
> > @@ -243,7 +244,7 @@ static int get_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  static int set_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  {
> >  	unsigned int max_vq, vq;
> > -	u64 vqs[DIV_ROUND_UP(SVE_VQ_MAX - SVE_VQ_MIN + 1, 64)];
> > +	u64 vqs[SVE_VLS_WORDS];
> >  
> >  	if (!vcpu_has_sve(vcpu))
> >  		return -ENOENT;
> > -- 
> > 2.1.4
> >
> 
> This is good, but I wonder if we could define the number of VLS words in
> the documentation in terms of SVE_VQ_MAX too. Currently it's just the
> hard coded 8 ("__u64 vector_lengths[8]").
> 
> Reviewed-by: Andrew Jones <drjones@redhat.com>

I see your point, but SVE_VQ_MAX isn't really part of the KVM API, so I
was avoiding it here.

[8] is at least impossible to misinterpret, even if it's not the most
self-explanatory option.

Is that OK?

Cheers
---Dave