Re: [Qemu-devel] [PATCH 1/1] s390/kvm: implement clearing part of IPL clear

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Thomas Huth (thuth@redhat.com) wrote:
> On 28.02.2018 20:53, Christian Borntraeger wrote:
> > When a guests reboots with diagnose 308 subcode 3 it requests the memory
> > to be cleared. We did not do it so far. This does not only violate the
> > architecture, it also misses the chance to free up that memory on
> > reboot, which would help on host memory over commitment.  By using
> > ram_block_discard_range we can cover both cases.
> 
> Sounds like a good idea. I wonder whether that release_all_ram()
> function should maybe rather reside in exec.c, so that other machines
> that want to clear all RAM at reset time can use it, too?
> 
> > Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > ---
> >  target/s390x/kvm.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> > index 8f3a422288..2e145ad5c3 100644
> > --- a/target/s390x/kvm.c
> > +++ b/target/s390x/kvm.c
> > @@ -34,6 +34,8 @@
> >  #include "qapi/error.h"
> >  #include "qemu/error-report.h"
> >  #include "qemu/timer.h"
> > +#include "qemu/rcu_queue.h"
> > +#include "sysemu/cpus.h"
> >  #include "sysemu/sysemu.h"
> >  #include "sysemu/hw_accel.h"
> >  #include "hw/boards.h"
> > @@ -41,6 +43,7 @@
> >  #include "sysemu/device_tree.h"
> >  #include "exec/gdbstub.h"
> >  #include "exec/address-spaces.h"
> > +#include "exec/ram_addr.h"
> >  #include "trace.h"
> >  #include "qapi-event.h"
> >  #include "hw/s390x/s390-pci-inst.h"
> > @@ -1841,6 +1844,14 @@ static int kvm_arch_handle_debug_exit(S390CPU *cpu)
> >      return ret;
> >  }
> >  
> > +static void release_all_rams(void)
> 
> s/rams/ram/ maybe?
> 
> > +{
> > +    struct RAMBlock *rb;
> > +
> > +    QLIST_FOREACH_RCU(rb, &ram_list.blocks, next)
> > +        ram_block_discard_range(rb, 0, rb->used_length);
> 
> From a coding style point of view, I think there should be curly braces
> around ram_block_discard_range() ?

I think this might break if it happens during a postcopy migrate.
The destination CPU is running, so it can do a reboot at just the wrong
time; and then the pages (that are protected by userfaultfd) would get
deallocated and trigger userfaultfd requests if accessed.

Dave

> > +}
> > +
> >  int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
> >  {
> >      S390CPU *cpu = S390_CPU(cs);
> > @@ -1853,6 +1864,14 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
> >              ret = handle_intercept(cpu);
> >              break;
> >          case KVM_EXIT_S390_RESET:
> > +            if (run->s390_reset_flags & KVM_S390_RESET_CLEAR) {
> > +                /*
> > +                 * We will stop other CPUs anyway, avoid spurious crashes and
> > +                 * get all CPUs out. The reset will take care of the resume.
> > +                 */
> > +                pause_all_vcpus();
> > +                release_all_rams();
> > +            }
> >              s390_reipl_request();
> >              break;
> >          case KVM_EXIT_S390_TSCH:
> > 
> 
> Apart from the cosmetic nits, patch looks good to me.
> 
>  Thomas
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK