RE: [PATCH RFC V2 31/37] physmem,gdbstub: Common helping funcs/changes to *unrealize* vCPU

[Salil Mehta]

Hi Phil,

> From: Philippe Mathieu-Daudé <philmd@linaro.org>
> Sent: Tuesday, October 3, 2023 7:34 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;
> eric.auger@redhat.com; will@kernel.org; ardb@kernel.org;
> oliver.upton@linux.dev; pbonzini@redhat.com; mst@redhat.com;
> gshan@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 31/37] physmem,gdbstub: Common helping
> funcs/changes to *unrealize* vCPU
> 
> Hi Salil,
> 
> On 26/9/23 12:04, Salil Mehta wrote:
> > Supporting vCPU Hotplug for ARM arch also means introducing new
> functionality of
> > unrealizing the ARMCPU. This requires some new common functions.
> >
> > Defining them as part of architecture independent change so that this
> code could
> > be reused by other interested parties.
> >
> > Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> > ---
> >   gdbstub/gdbstub.c         | 13 +++++++++++++
> >   include/exec/cpu-common.h |  8 ++++++++
> >   include/exec/gdbstub.h    |  1 +
> >   include/hw/core/cpu.h     |  1 +
> >   softmmu/physmem.c         | 25 +++++++++++++++++++++++++
> >   5 files changed, 48 insertions(+)
> 
> 
> > diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> > index dab572c9bd..ffd815a0d8 100644
> > --- a/include/hw/core/cpu.h
> > +++ b/include/hw/core/cpu.h
> > @@ -366,6 +366,7 @@ struct CPUState {
> >       QSIMPLEQ_HEAD(, qemu_work_item) work_list;
> >
> >       CPUAddressSpace *cpu_ases;
> > +    int cpu_ases_ref_count;
> >       int num_ases;
> >       AddressSpace *as;
> >       MemoryRegion *memory;
> > diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> > index 3df73542e1..a93ae783af 100644
> > --- a/softmmu/physmem.c
> > +++ b/softmmu/physmem.c
> > @@ -762,6 +762,7 @@ void cpu_address_space_init(CPUState *cpu, int asidx,
> >
> >       if (!cpu->cpu_ases) {
> >           cpu->cpu_ases = g_new0(CPUAddressSpace, cpu->num_ases);
> > +        cpu->cpu_ases_ref_count = cpu->num_ases;
> >       }
> >
> >       newas = &cpu->cpu_ases[asidx];
> > @@ -775,6 +776,30 @@ void cpu_address_space_init(CPUState *cpu, int
> asidx,
> >       }
> >   }
> >
> > +void cpu_address_space_destroy(CPUState *cpu, int asidx)
> > +{
> > +    CPUAddressSpace *cpuas;
> > +
> > +    assert(asidx < cpu->num_ases);
> > +    assert(asidx == 0 || !kvm_enabled());
> > +    assert(cpu->cpu_ases);
> > +
> > +    cpuas = &cpu->cpu_ases[asidx];
> > +    if (tcg_enabled()) {
> > +        memory_listener_unregister(&cpuas->tcg_as_listener);
> > +    }
> > +
> > +    address_space_destroy(cpuas->as);
> > +    g_free_rcu(cpuas->as, rcu);
> > +
> > +    if (cpu->cpu_ases_ref_count == 1) {
> > +        g_free(cpu->cpu_ases);
> > +        cpu->cpu_ases = NULL;
> > +    }
> > +
> > +    cpu->cpu_ases_ref_count--;
> 
> See Richard comment from:
> https://lore.kernel.org/qemu-devel/594b2550-9a73-684f-6e54-
> 29401dc6cd7a@linaro.org/
> 
> "I think it would be better to destroy all address spaces at once,
> "so that you don't need  to invent a reference count that isn't used
> "for anything else.

Yes, we can do that and remove the reference count. The only reason I
did it was because I was not sure if it is safe to assume that all
the AddressSpace will always be destroyed *together*. And now since
this is being ported to other architectures will the same hold
true everywhere?


Thanks
Salil.