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

Dave Martin
2019-04-05T09:36:03Z

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.

>
> > +
> > + 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