On Thu, Nov 22, 2018 at 11:13:51AM +0000, Peter Maydell wrote:
> On 22 November 2018 at 10:53, Christoffer Dall <[email protected]>
> wrote:
> > [Adding Peter and Alex for their view on the QEMU side]
> >
> > On Thu, Nov 15, 2018 at 05:27:11PM +0000, Dave Martin wrote:
> >> My expectation is that KVM_ARM_SVE_CONFIG_SET is considered part of
> >> creating the vcpu, so that if issued at all for a vcpu, it is issued
> >> very soon after KVM_VCPU_INIT.
> >>
> >> I think this worked OK with the current structure of kvmtool and I
> >> seem to remember discussing this with Peter Maydell re qemu -- but
> >> it sounds like I should double-check.
> >
> > QEMU does some thing around enumerating all the system registers exposed
> > by KVM and saving/restoring them as part of its startup, but I don't
> > remember the exact sequence.
>
> This all happens in kvm_arch_init_vcpu(), which does:
> * KVM_ARM_VCPU_INIT ioctl (with the appropriate kvm_init_features set)
> * read the guest MPIDR with GET_ONE_REG so we know what KVM
> is doing with MPIDR assignment across CPUs
> * check for interesting extensions like KVM_CAP_SET_GUEST_DEBUG
> * get and cache a list of what system registers the vcpu has,
> using KVM_GET_REG_LIST. This is where we do the "size must
> be U32 or U64" sanity check.
>
> So if there's something we can't do by setting kvm_init_features
> for KVM_ARM_VCPU_INIT but have to do immediately afterwards,
> that is straightforward.
>
> The major requirement for QEMU is that if we don't specifically
> enable SVE in the VCPU then we must not see any registers
> in the KVM_GET_REG_LIST that are not u32 or u64 -- otherwise
> QEMU will refuse to start.
>
So on migration, will you have the required information for
KVM_ARM_VCPU_INIT before setting the registers from the migration
stream?
(I assume so, because presumably this comes from a command-line switch
or from the machine definition, which must match the source.)
Therefore, I don't think there's an issue with this patch, but from
bitter experience I think we should enforce ordering if possible.
Thanks,
Christoffer
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