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

[Salil Mehta]

Hi Phil/Richard,

> From: qemu-arm-bounces+salil.mehta=huawei.com@nongnu.org <qemu-arm-
> bounces+salil.mehta=huawei.com@nongnu.org> On Behalf Of Salil Mehta via
> Sent: Tuesday, October 3, 2023 11:23 AM
> 
> 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?

(sorry, I missed key point)

To make things clear further for ARM, presence of tagged/secure memory
is optional (and I am not even sure all of these are supported with
accel=KVM). The Address Space destruction function is common to all
architectures and hence it is not safe to destroy all of these together.

https://lore.kernel.org/qemu-devel/20230926100436.28284-1-salil.mehta@huawei.com/T/#mfb2a525081c412917a0026d558e72f48875e386d

+static void arm_cpu_unrealizefn(DeviceState *dev)
+{
+    ARMCPUClass *acc = ARM_CPU_GET_CLASS(dev);
+    ARMCPU *cpu = ARM_CPU(dev);
+    CPUARMState *env = &cpu->env;
+    CPUState *cs = CPU(dev);
+    bool has_secure;
+
+    has_secure = cpu->has_el3 || arm_feature(env, ARM_FEATURE_M_SECURITY);
+
+    /* rock 'n' un-roll, whatever happened in the arm_cpu_realizefn cleanly */
+    cpu_address_space_destroy(cs, ARMASIdx_NS);
+
+    if (cpu->tag_memory != NULL) {
+        cpu_address_space_destroy(cs, ARMASIdx_TagNS);
+        if (has_secure) {
+            cpu_address_space_destroy(cs, ARMASIdx_TagS);
+        }
+    }
+
+    if (has_secure) {
+        cpu_address_space_destroy(cs, ARMASIdx_S);
+    }
 [...]
}



@Richard, please let me know if I understood your comment correctly?


Thanks
Salil.