On Wed, Feb 07, 2018 at 03:34:03PM +0000, Dave Martin wrote:
> On Wed, Feb 07, 2018 at 03:58:31PM +0100, Christoffer Dall wrote:
> > On Wed, Feb 07, 2018 at 11:33:28AM +0000, Dave Martin wrote:
>
> [...]
>
> > > What if KVM_ARM_SVE_SET_VLS() were to yield 0 if the exact requested set
> > > of VLs was configured, -ERANGE if some subset was configured successfully
> > > (but not exactly the set requested), and the usual -EINVAL/-EIO/whatever
> > > if the set of VLs couldn't be configured at all?
> >
> > Sounds good to me.
> >
> > >
> > > Then the probe would go like this:
> > >
> > > __u64 vqs[SVE_VQ_MAX / 64] = { [0 ... SVE_VQ_MAX / 64 - 1] = ~(u64)0 };
> > > if (ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs) && errno != ERANGE))
> > > goto error;
> > >
> > > ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
> > >
> > > /* ... */
> > >
> > > Another option would be for SVE_ARM_SVE_SET_VLS to write the resulting
> > > set back to its argument, possibly with the same 0/ERANGE/EINVAL semantics.
> > >
> > >
> > > Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS,
> > > and check the resulting set:
> > >
> > > /* ... */
> > >
> > > __u64 newvqs[SVE_VQ_MAX / 64];
> > > ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs);
> > >
> > > if (memcmp(vqs, newvqs, sizeof vqs))
> > > goto mismatch;
> > >
> > > vcpu restore would need to treat any mismatch as an error:
> > > the exact requested set but be configurable, or the VM may go wrong.
> >
> > I'm not sure I can parse this sentence or extract the meaning?
>
> That was lazy language on my part. I'll try to explain it better:
>
> When the saved state of a migrating vcpu is being loaded into a
> newly-created vcpu on the target node, userspace needs a way to
> ensure that the set of VLs that vcpu will see when it runs is
> _exactly_ the same set it could see before migration.
Yes, and that should work fine with the -ERANGE proposal, right?
>
> I called this out separately because it's different from the
> case of creating a brand-new VM: in the latter case, we can't the
> kernel to provide the best set of VLs possible, but it is not an
> error not to get every VL we asked for.
>
This was another hard one:)
I think what you're saying is that it's technically not an error when
setting the VLs on a new VM, but it would be in the case of migration,
and therefore we need to tell userspace in both cases what happened, and
it can decide.
> > > Any opinion on which approach is best?
> >
> > I think I prefer letting KVM_ARM_SVE_SET_VLS change the supplied vqs,
> > since having it be two separate ioctls always potentially leaves room
> > for some other thread having modified the set in the meantime (or making
> > a programmer doubt if this can be the case) where a single ioctl() will
> > look atomic.
> >
> > The user can always call KVM_ARM_SVE_GET_VLS afterwards and should get
> > the same result.
>
> OK, I'll go with changing the supplied vqs for KVM_ARM_SVE_SET_VLS,
> but I'll retain the -ERANGE semantics (even if technically redundant)
> since that's harder to forget to check.
Agree, that should definitely be part of it.
Thanks,
-Christoffer
--
libvir-list mailing list
libvir-list@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/libvir-list
