Re: [PATCH v10 04/18] KVM: arm/arm64: Introduce kvm_arch_vcpu_run_pid_change

["Alex Bennée" <alex.bennee@linaro.org>]

Christoffer Dall <christoffer.dall@arm.com> writes:

> On Wed, May 23, 2018 at 03:40:26PM +0100, Dave Martin wrote:
>> On Wed, May 23, 2018 at 03:34:20PM +0100, Alex Bennée wrote:
>> >
>> > Dave Martin <Dave.Martin@arm.com> writes:
>> >
>> > > From: Christoffer Dall <christoffer.dall@linaro.org>
>> > >
>> > > KVM/ARM differs from other architectures in having to maintain an
>> > > additional virtual address space from that of the host and the
>> > > guest, because we split the execution of KVM across both EL1 and
>> > > EL2.
>> > >
>> > > This results in a need to explicitly map data structures into EL2
>> > > (hyp) which are accessed from the hyp code.  As we are about to be
>> > > more clever with our FPSIMD handling on arm64, which stores data in
>> > > the task struct and uses thread_info flags, we will have to map
>> > > parts of the currently executing task struct into the EL2 virtual
>> > > address space.
>> > >
>> > > However, we don't want to do this on every KVM_RUN, because it is a
>> > > fairly expensive operation to walk the page tables, and the common
>> > > execution mode is to map a single thread to a VCPU.  By introducing
>> > > a hook that architectures can select with
>> > > HAVE_KVM_VCPU_RUN_PID_CHANGE, we do not introduce overhead for
>> > > other architectures, but have a simple way to only map the data we
>> > > need when required for arm64.
>> > >
>> > > This patch introduces the framework only, and wires it up in the
>> > > arm/arm64 KVM common code.
>> > >
>> > > No functional change.
>> > >
>> > > Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
>> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
>> > > Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
>> > > ---
>> > >  include/linux/kvm_host.h | 9 +++++++++
>> > >  virt/kvm/Kconfig         | 3 +++
>> > >  virt/kvm/kvm_main.c      | 7 ++++++-
>> > >  3 files changed, 18 insertions(+), 1 deletion(-)
>> > >
>> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> > > index 6930c63..4268ace 100644
>> > > --- a/include/linux/kvm_host.h
>> > > +++ b/include/linux/kvm_host.h
>> > > @@ -1276,4 +1276,13 @@ static inline long kvm_arch_vcpu_async_ioctl(struct file *filp,
>> > >  void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>> > >  		unsigned long start, unsigned long end);
>> > >
>> > > +#ifdef CONFIG_HAVE_KVM_VCPU_RUN_PID_CHANGE
>> > > +int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu);
>> > > +#else
>> > > +static inline int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
>> > > +{
>> > > +	return 0;
>> > > +}
>> > > +#endif /* CONFIG_HAVE_KVM_VCPU_RUN_PID_CHANGE */
>> > > +
>> > >  #endif
>> > > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
>> > > index cca7e06..72143cf 100644
>> > > --- a/virt/kvm/Kconfig
>> > > +++ b/virt/kvm/Kconfig
>> > > @@ -54,3 +54,6 @@ config HAVE_KVM_IRQ_BYPASS
>> > >
>> > >  config HAVE_KVM_VCPU_ASYNC_IOCTL
>> > >         bool
>> > > +
>> > > +config HAVE_KVM_VCPU_RUN_PID_CHANGE
>> > > +       bool
>> >
>> > This almost threw me as I thought you might be able to enable this and
>> > break the build, but apparently not:
>> >
>> > Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>>
>> Without a "help", the option seems non-interactive and cannot be true
>> unless something selects it.  It seems a bit weird to me too, but the
>> idiom appears widely used...
>>
> Indeed, I've copied this idiom from other things before and nobody has
> complained, so I think it works (without any further deep insights into
> the inner workings of Kconfig).

It's fine. My main worry was breaking bisection with the normal "make
olddefconfig" approach. I tested it and found it to be fine and I don't
think we need to worry about people adding the symbol to .config
manually - they get to keep both pieces ;-)

--
Alex Bennée

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