On Mon, May 13, 2019 at 01:30:23PM +0100, Andrew Jones wrote:
> On Mon, May 13, 2019 at 12:26:36PM +0100, Dave Martin wrote:
> > On Sun, May 12, 2019 at 09:36:22AM +0100, Andrew Jones wrote:
> > > Introduce another cpu property to control SVE vector lengths,
> > > sve-vls-map, which allows the user to explicitly select the
> > > set of vector lengths the guest can use. The map must conform
> > > to QEMU's limits and architectural constraints, checked when
> > > the property is set. Inconsistencies with sve-max-vq are also
> > > checked. The bit number of a set bit in the map represents the
> > > allowed vector length in number of quadwords.
> > >
> > > Note, as the map is implemented with a single 64-bit word we
> > > currently only support up to 8192-bit vectors. As QEMU and
> > > KVM only support up to 2048-bit vectors then this sufficient
> > > now, and probably for some time. Extending the bitmap beyond
> > > a single word will likely require changing the property to
> > > a string and adding yet another parser to QEMU.
> > >
> > > Signed-off-by: Andrew Jones <drjo...@redhat.com>
> > > ---
> > > target/arm/cpu.c | 4 +++
> > > target/arm/cpu.h | 3 ++
> > > target/arm/cpu64.c | 70 +++++++++++++++++++++++++++++++++++++++++---
> > > target/arm/helper.c | 10 ++++++-
> > > target/arm/monitor.c | 9 ++++--
> > > 5 files changed, 88 insertions(+), 8 deletions(-)
> > >
> >
> > [...]
> >
> > > diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> > > index 8292d547e8f9..f0d0ce759ba8 100644
> > > --- a/target/arm/cpu.h
> > > +++ b/target/arm/cpu.h
> > > @@ -920,6 +920,9 @@ struct ARMCPU {
> > >
> > > /* Used to set the maximum vector length the cpu will support. */
> > > uint32_t sve_max_vq;
> > > +
> > > + /* Each bit represents a supported vector length of (bitnum * 16)
> > > bytes */
> > > + uint64_t sve_vls_map;
> >
> > Just to be clear, the representation here is different from the
> > representation in KVM_REG_ARM64_SVE_VLS?
> >
> > In the latter, bit n represents vector length ((n + 1) * 16) bytes.
>
> KVM also uses bitnum * 16. bitnum is the the bit number, not the shift.
Can you point to the relevant kernel code? This isn't what I thought I
wrote...
> So you can't have bitnum=0. 'n' must be a shift, as it would need to
> allow zero in order to represent KVM_ARM64_SVE_VQ_MIN.
[...]
> > > +static void cpu_set_sve_vls_map(Object *obj, Visitor *v, const char
> > > *name,
> > > + void *opaque, Error **errp)
> > > +{
> > > + ARMCPU *cpu = ARM_CPU(obj);
> > > + Error *err = NULL;
> > > + uint64_t mask = ~(BIT_MASK(ARM_MAX_VQ - 1) - 1);
> > > + int i;
> > > +
> > > + visit_type_uint64(v, name, &cpu->sve_vls_map, errp);
> > > +
> > > + if (!err) {
> > > + if (cpu->sve_vls_map == 0) {
> > > + error_setg(&err, "SVE vector length map cannot be zero");
> >
> > Maybe say "empty" here, since the map represents a set?
>
> Empty does sound better when considering the map represents a set, but
> the user will be inputting a number for the map, so zero will be what
> they attempted to input.
If the user is inputting the encoded number directly, I guess it makes
sense to describe the empty set as "zero".
> > (But it this is just for debug rather than reporting errors to the user,
> > it probably doesn't matter much.)
>
> I'm also not too worried about which term we use, so I'm happy to change
> it if you'd like.
I guess I don't have a strong opinion.
[...]
> > > diff --git a/target/arm/helper.c b/target/arm/helper.c
> > > index 1e6eb0d0f360..bedec1ea0b27 100644
> > > --- a/target/arm/helper.c
> > > +++ b/target/arm/helper.c
> > > @@ -5254,12 +5254,20 @@ uint32_t sve_zcr_len_for_el(CPUARMState *env, int
> > > el)
> > > static void zcr_write(CPUARMState *env, const ARMCPRegInfo *ri,
> > > uint64_t value)
> > > {
> > > + ARMCPU *cpu = arm_env_get_cpu(env);
> > > int cur_el = arm_current_el(env);
> > > int old_len = sve_zcr_len_for_el(env, cur_el);
> > > int new_len;
> > >
> > > /* Bits other than [3:0] are RAZ/WI. */
> > > - raw_write(env, ri, value & 0xf);
> > > + value &= 0xf;
> >
> > You might want to sanity-check that the max vq you configured for the
> > vcpu is <= 16 here.
>
> It's already checked by the cpu property set function at input time.
The check is in one place, but the assumptions based on it are
potentially all over the place -- it may be tricky to track them
all down if you ever add support for >8192-bit vectors.
(Not a big deal from my point of view, but I'd migration to larger
vectors to be as smooth as reasonably possible if we ever find we have
to do it.)
Cheers
---Dave
