RE: [PATCH RFC V2 07/37] arm/virt, gicv3: Changes to pre-size GIC with possible vcpus @machine init

[Salil Mehta]

Hi Gavin,

> From: Gavin Shan <gshan@redhat.com>
> Sent: Thursday, September 28, 2023 1:14 AM
> To: Salil Mehta <salil.mehta@huawei.com>; qemu-devel@nongnu.org; 
> qemu-arm@nongnu.org
> Cc: maz@kernel.org; jean-philippe@linaro.org; Jonathan Cameron
> <jonathan.cameron@huawei.com>; lpieralisi@kernel.org;
> peter.maydell@linaro.org; richard.henderson@linaro.org;
> imammedo@redhat.com; andrew.jones@linux.dev; david@redhat.com;
> philmd@linaro.org; eric.auger@redhat.com; will@kernel.org; ardb@kernel.org;
> oliver.upton@linux.dev; pbonzini@redhat.com; mst@redhat.com;
> rafael@kernel.org; borntraeger@linux.ibm.com; alex.bennee@linaro.org;
> linux@armlinux.org.uk; darren@os.amperecomputing.com;
> ilkka@os.amperecomputing.com; vishnu@os.amperecomputing.com;
> karl.heubaum@oracle.com; miguel.luis@oracle.com; salil.mehta@opnsrc.net;
> zhukeqian <zhukeqian1@huawei.com>; wangxiongfeng (C)
> <wangxiongfeng2@huawei.com>; wangyanan (Y) <wangyanan55@huawei.com>;
> jiakernel2@gmail.com; maobibo@loongson.cn; lixianglai@loongson.cn
> Subject: Re: [PATCH RFC V2 07/37] arm/virt,gicv3: Changes to pre-size GIC
> with possible vcpus @machine init
> 
> Hi Salil,
> 
> On 9/26/23 20:04, Salil Mehta wrote:
> > GIC needs to be pre-sized with possible vcpus at the initialization time. 
> > This
> > is necessary because Memory regions and resources associated with GICC/GICR
> > etc cannot be changed (add/del/modified) after VM has inited. Also, 
> > GIC_TYPER
> > needs to be initialized with mp_affinity and cpu interface number 
> > association.
> > This cannot be changed after GIC has initialized.
> >
> > Once all the cpu interfaces of the GIC has been inited it needs to be 
> > ensured
>                                                    ^^^^^^
>                                                    initialized,

Sure. Thanks!


> > that any updates to the GICC during reset only takes place for the present
> 
> ^^^^^^^^^^^
>                                                                   the
> enabled


Yes. I will fix the sentence.

Thanks
Salil.


> > vcpus and not the disabled ones. Therefore, proper checks are required at
> > various places.
> >
> > Co-developed-by: Salil Mehta <salil.mehta@huawei.com>
> > Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> > Co-developed-by: Keqian Zhu <zhukeqian1@huawei.com>
> > Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > [changed the comment in arm_gicv3_icc_reset]
> > Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> > ---
> >   hw/arm/virt.c              | 15 ++++++++-------
> >   hw/intc/arm_gicv3_common.c |  7 +++++--
> >   hw/intc/arm_gicv3_cpuif.c  |  8 ++++++++
> >   hw/intc/arm_gicv3_kvm.c    | 34 +++++++++++++++++++++++++++++++---
> >   include/hw/arm/virt.h      |  2 +-
> >   5 files changed, 53 insertions(+), 13 deletions(-)
> >
> 
> I guess the subject can be improved to something like below because it's the 
> preparatory
> work to support vCPU hotplug (notifier) in the subsequent patches. In this 
> patch, most
> of the code changes is related to vCPU state, ms->smp_pros.max_cpus and the 
> CPU interface
> instances associated to GICv3 controller.

I general, any commit-log should state 'why' and not 'what' the patch is doing.

> 
> arm/virt,gicv3: Prepare for vCPU hotplug by checking GICv3CPUState states

Subject is a summary of what patch intends to do. It is pre-sizing the GIC in 
QOM
and in KVM as well i.e. VGIC. Please check the KVMForum 2023 slides page-6 for 
more
details.


> We already had wrappers to check vCPU's states. I'm wandering if we need 
> another set
> of wrappers for GICv3 for several facts: (a) In this patch, we're actually 
> concerned
> by GICv3CPUState's states, disabled or enabled. vCPU states have been 
> classified to
> possible, present, and enabled. Their states aren't matching strictly. (b) 
> With GICv3
> own wrappers, the code can be detached from vCPU in high level. Please 
> evaluate it's
> worthy to have GICv3 own wrappers and we can have the folowing wrappers if
> want.

It will open up the pandoras box if we tickle with GIC as we cannot play with 
VGIC
states in the KVM due to ARM architecture constraints. These states are always 
part
of always-on power domain. In case, we decide to model the QEMU GIC to relax 
later
aspect we would still need to re-size the VGIC at KVM and still expose all of 
the
GIC CPU Interfaces to the Guest OS at boot time. Hence, those GICV3CPUState 
specific
APIs do not add much value to logic to be frank.

Plus, once we have these states formally described as part of the QOM then we 
need
to explicitly handle cases, for example, what will these new states mean in 
terms
of Live/Pseudo Migration?


Thanks
Salil.


[...]

> > diff --git a/hw/intc/arm_gicv3_common.c b/hw/intc/arm_gicv3_common.c
> > index 2ebf880ead..ebd99af610 100644
> > --- a/hw/intc/arm_gicv3_common.c
> > +++ b/hw/intc/arm_gicv3_common.c
> > @@ -392,10 +392,13 @@ static void arm_gicv3_common_realize(DeviceState
> *dev, Error **errp)
> >       s->cpu = g_new0(GICv3CPUState, s->num_cpu);
> >
> >       for (i = 0; i < s->num_cpu; i++) {
> > -        CPUState *cpu = qemu_get_cpu(i);
> > +        CPUState *cpu = qemu_get_possible_cpu(i);
> >           uint64_t cpu_affid;
> >
> > -        s->cpu[i].cpu = cpu;
> > +        if (qemu_enabled_cpu(cpu)) {
> > +            s->cpu[i].cpu = cpu;
> > +        }
> > +
> >           s->cpu[i].gic = s;
> >           /* Store GICv3CPUState in CPUARMState gicv3state pointer */
> >           gicv3_set_gicv3state(cpu, &s->cpu[i]);
> 
> I don't think gicv3_set_gicv3state() isn't needed for !qemu_enabled_cpu(cpu)
> since those disabled vCPUs will be released in hw/arm/virt.c pretty soon.


For disabled CPUs, GICV3CPUState will be initialized to NULL and will
continue to exists even after its corresponding QOM CPUState objects
have been released.


> > diff --git a/hw/intc/arm_gicv3_cpuif.c b/hw/intc/arm_gicv3_cpuif.c
> > index d07b13eb27..7b7a0fdb9c 100644
> > --- a/hw/intc/arm_gicv3_cpuif.c
> > +++ b/hw/intc/arm_gicv3_cpuif.c
> > @@ -934,6 +934,10 @@ void gicv3_cpuif_update(GICv3CPUState *cs)
> >       ARMCPU *cpu = ARM_CPU(cs->cpu);
> >       CPUARMState *env = &cpu->env;
> >
> > +    if (!qemu_enabled_cpu(cs->cpu)) {
> > +        return;
> > +    }
> > +
> 
> The question is how it's possible. It seems a bug to update GICv3CPUState
> who isn't ready or disabled.

Ideally, it should not. This code is meant for TCG and any updates happening
for the entire GICv3 like in GICv3_update() function, which iterates over number
of GICV3 CPUs, this is required to ensure updates do not happen if corresponding
CPUState object does not exist.

[...]

> >
> > +    /*
> > +     * This shall be called even when vcpu is being hotplugged or onlined 
> > and
> > +     * other vcpus might be running. Host kernel KVM code to handle device
> > +     * access of IOCTLs KVM_{GET|SET}_DEVICE_ATTR might fail due to 
> > inability to
> > +     * grab vcpu locks for all the vcpus. Hence, we need to pause all 
> > vcpus to
> > +     * facilitate locking within host.
> > +     */
> > +    pause_all_vcpus();
> >       /* Initialize to actual HW supported configuration */
> >       kvm_device_access(s->dev_fd, KVM_DEV_ARM_VGIC_GRP_CPU_SYSREGS,
> >                         KVM_VGIC_ATTR(ICC_CTLR_EL1, c->gicr_typer),
> >                         &c->icc_ctlr_el1[GICV3_NS], false, &error_abort);
> > +    resume_all_vcpus();
> 
> Please swap the positions for paused_all_vcpu() and the next comment, and
> then combine the comments.


No issues.


Thanks
Salil.