Re: [PATCH v7 19/27] KVM: arm64: Enumerate SVE register indices for KVM_GET_REG_LIST

Andrew Jones
2019-04-05T09:46:14Z

On Fri, Apr 05, 2019 at 10:35:45AM +0100, Dave Martin wrote:
> On Thu, Apr 04, 2019 at 04:08:32PM +0200, Andrew Jones wrote:
> > On Fri, Mar 29, 2019 at 01:00:44PM +0000, Dave Martin wrote:
> > > 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 new IDs are added.
> > >
> > > On an SVE-enabled vcpu, IDs for the FPSIMD V-registers are removed
> > > from the list, since userspace is required to access the Z-
> > > registers instead in order to access the V-register content. For
> > > the variably-sized SVE registers, the appropriate set of slice IDs
> > > are enumerated, depending on the maximum vector length for the
> > > vcpu.
> > >
> > > As it currently stands, the SVE architecture never requires more
> > > than one slice to exist per register, so this patch adds no
> > > explicit support for enumerating multiple slices. The code can be
> > > extended straightforwardly to support this in the future, if
> > > needed.
> > >
> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > > Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> > > Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> > >
> > > ---
> > >
> > > Changes since v6:
> > >
> > > * [Julien Thierry] Add a #define to replace the magic "slices = 1",
> > > and add a comment explaining to maintainers what needs to happen if
> > > this is updated in the future.
> > >
> > > Changes since v5:
> > >
> > > (Dropped Julien Thierry's Reviewed-by due to non-trivial rebasing)
> > >
> > > * Move mis-split reword to prevent put_user()s being accidentally the
> > > correct size from KVM: arm64/sve: Add pseudo-register for the guest's
> > > vector lengths.
> > > ---
> > > arch/arm64/kvm/guest.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 63 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > > index 736d8cb..2aa80a5 100644
> > > --- a/arch/arm64/kvm/guest.c
> > > +++ b/arch/arm64/kvm/guest.c
> > > @@ -222,6 +222,13 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> > > #define KVM_SVE_ZREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0))
> > > #define KVM_SVE_PREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_PREG(0, 0))
> > >
> > > +/*
> > > + * number of register slices required to cover each whole SVE register on vcpu
> >
> > s/number/Number/
>
> Not a sentence -> no capital letter.
>
> Due to the adjacent note it does look a little odd though. I'm happy to
> change it.
>
> > s/on vcpu//
>
> Agreed, I can drop that.
>
> > > + * NOTE: If you are tempted to modify this, you must also to rework
> >
> > s/to rework/rework/
>
> Ack
>
> > > + * sve_reg_to_region() to match:
> > > + */
> > > +#define vcpu_sve_slices(vcpu) 1
> > > +
> > > /* Bounds of a single SVE register slice within vcpu->arch.sve_state */
> > > struct sve_state_reg_region {
> > > unsigned int koffset; /* offset into sve_state in kernel memory */
> > > @@ -411,6 +418,56 @@ 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)
> > > +{
> > > + /* Only the first slice ever exists, for now */
> >
> > I'd move this comment up into the one above vcpu_sve_slices(),
> > and then nothing needs to change here when more slices come.
> >
> > > + const unsigned int slices = vcpu_sve_slices(vcpu);
> > > +
> > > + 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 *uindices)
> > > +{
> > > + /* Only the first slice ever exists, for now */
> >
> > Same comment as above.
>
> Fair point: this was to explain the magic "1" that was previously here,
> but the comments are a bit redundant here now: better to move the
> comments where the 1 itself went.
>
> > > + const unsigned int slices = vcpu_sve_slices(vcpu);
> > > + u64 reg;
> > > + unsigned int i, n;
> > > + int num_regs = 0;
> > > +
> > > + if (!vcpu_has_sve(vcpu))
> > > + return 0;
> > > +
> > > + for (i = 0; i < slices; i++) {
> > > + for (n = 0; n < SVE_NUM_ZREGS; n++) {
> > > + reg = KVM_REG_ARM64_SVE_ZREG(n, i);
> > > + if (put_user(reg, uindices++))
> > > + return -EFAULT;
> > > +
> > > + num_regs++;
> > > + }
> > > +
> > > + for (n = 0; n < SVE_NUM_PREGS; n++) {
> > > + reg = KVM_REG_ARM64_SVE_PREG(n, i);
> > > + if (put_user(reg, uindices++))
> > > + return -EFAULT;
> > > +
> > > + num_regs++;
> > > + }
> > > +
> > > + reg = KVM_REG_ARM64_SVE_FFR(i);
> > > + if (put_user(reg, uindices++))
> > > + return -EFAULT;
> > > +
> > > + num_regs++;
> > > + }
> >
> > nit: the extra blank lines above the num_regs++'s give the code an odd
> > look (to me)
>
> There's no guaranteed fall-through onto the increments: the blank line
> was there to highlight the fact that we may jump out using a return
> instead.
>
> But I'm happy enough to change it if you have a strong preference or
> you feel the code is equally clear without.

It's just a nit, so I don't have a strong preference :)

>
> >
> > > +
> > > + return num_regs;
> > > +}
> > > +
> > > /**
> > > * kvm_arm_num_regs - how many registers do we present via KVM_GET_ONE_REG
> > > *
> > > @@ -421,6 +478,7 @@ unsigned long kvm_arm_num_regs(struct kvm_vcpu *vcpu)
> > > unsigned long res = 0;
> > >
> > > res += num_core_regs(vcpu);
> > > + 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;
> > > @@ -442,6 +500,11 @@ int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
> > > return ret;
> > > uindices += ret;
> > >
> > > + ret = copy_sve_reg_indices(vcpu, uindices);
> > > + if (ret)
> > > + return ret;
> > > + uindices += ret;
> >
> > I know this if ret vs. if ret < 0 is being addressed already.
>
> Yes, Marc's patch in kvmarm/next should fix that.
>
> Cheers
> ---Dave

Thanks,
drew