Re: [PATCH v6 05/10] KVM: arm64: Support stolen time reporting via shared structure

[Marc Zyngier]

On 2019-10-21 11:21, Steven Price wrote:
> On 19/10/2019 12:12, Marc Zyngier wrote:
> > On Fri, 11 Oct 2019 13:59:25 +0100,
> > Steven Price <steven.price@xxxxxxx> wrote:
> > > Implement the service call for configuring a shared structure 
> > > between a
> > > VCPU and the hypervisor in which the hypervisor can write the time
> > > stolen from the VCPU's execution time by other tasks on the host.
> > >
> > > User space allocates memory which is placed at an IPA also chosen 
> > > by user
> > > space. The hypervisor then updates the shared structure using
> > > kvm_put_guest() to ensure single copy atomicity of the 64-bit value
> > > reporting the stolen time in nanoseconds.
> > >
> > > Whenever stolen time is enabled by the guest, the stolen time 
> > > counter is
> > > reset.
> > >
> > > The stolen time itself is retrieved from the sched_info structure
> > > maintained by the Linux scheduler code. We enable SCHEDSTATS when
> > > selecting KVM Kconfig to ensure this value is meaningful.
> > >
> > > Signed-off-by: Steven Price <steven.price@xxxxxxx>
> > > ---
> > >  arch/arm/include/asm/kvm_host.h   | 20 +++++++++++
> > >  arch/arm64/include/asm/kvm_host.h | 21 +++++++++++-
> > >  arch/arm64/kvm/Kconfig            |  1 +
> > >  include/linux/kvm_types.h         |  2 ++
> > >  virt/kvm/arm/arm.c                | 11 ++++++
> > >  virt/kvm/arm/hypercalls.c         |  3 ++
> > >  virt/kvm/arm/pvtime.c             | 56 
> > > +++++++++++++++++++++++++++++++
> > >  7 files changed, 113 insertions(+), 1 deletion(-)

[...]

> > > +long kvm_hypercall_stolen_time(struct kvm_vcpu *vcpu)
> >
> > Why long? If that's a base address, then it is either a phys_addr_t 
> > or
> > a gpa_t. I'd suggest you move the error check to the caller.
>
> This is a bit more tricky. It's a long because that's the declared 
> type
> of the SMCCC return in kvm_hvc_call_handler(). I can't (easily) move 
> the
> code into kvm_hvc_call_handler() because that is compiled for arm (as
> well as arm64) and we don't have the definitions for stolen time 
> there.
> The best option I could come up with is to have a dummy stub for arm 
> and
> use generic types for this function.
>
> This means we need a type which can contain both a gpa_t and the
> SMCCC_RET_NOT_SUPPORTED error code.
>
> I'm open to alternative suggestions on how to make this work.

My suggestion would be to always return a gpa_t from this function, and
change the 32bit stub for kvm_hypercall_stolen_time() to always return
GPA_INVALID.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...