Re: [PATCH v1 1/1] s390x/pv: Fix spurious warning with asynchronous teardown

[Claudio Imbrenda]

On Wed, 10 May 2023 08:47:08 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On 09/05/2023 18.27, Claudio Imbrenda wrote:
> > When rebooting a small VM using asynchronous teardown, a spurious
> > warning is emitted when the KVM_PV_ASYNC_CLEANUP_PREPARE ioctl fails.  
> 
> Why does the _PREPARE fail in that case? Why 4GiB and not more or less? This 

because of kernel commit 292a7d6fca33df70ca4b8e9b0d0e74adf87582dc, which
fixes problems in case the VM is small (<2GiB)

> sounds racy... what if you have a faster or slower machine?

why racy?

2 or 4GiB is still very fast, and at some point you have to draw a line.
I could make it 2GiB, which is the limit at which _PREPARE will fail,
but since I'm touching this code, I would like to avoid unnecessary
overhead, instead of "just fixing" 

I can put the limit to 2GiB if you think it's more clean

> 
> > Avoid using asynchronous teardown altogether when the VM is small
> > enough; the cutoff is set at 4GiB. This will avoid triggering the
> > warning and also avoid pointless overhead; normal teardown is fast
> > enough for small VMs.
> > 
> > Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
> > Fixes: c3a073c610 ("s390x/pv: Add support for asynchronous teardown for 
> > reboot")
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > ---
> >   hw/s390x/pv.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
> > index 49ea38236c..17c5556319 100644
> > --- a/hw/s390x/pv.c
> > +++ b/hw/s390x/pv.c
> > @@ -13,6 +13,7 @@
> >   
> >   #include <linux/kvm.h>
> >   
> > +#include "qemu/units.h"
> >   #include "qapi/error.h"
> >   #include "qemu/error-report.h"
> >   #include "sysemu/kvm.h"
> > @@ -117,13 +118,16 @@ static void *s390_pv_do_unprot_async_fn(void *p)
> >   
> >   bool s390_pv_vm_try_disable_async(void)
> >   {
> > +    MachineState *machine = MACHINE(qdev_get_machine());  
> 
> The calling function (s390_machine_unprotect()) already has a 
> S390CcwMachineState as parameter ... so you could pass along that value to 
> avoid the qdev_get_machine() here.

yes, I was thinking about that and decided against it to avoid changing
interfaces; I'll fix it in the next version

> 
> >       /*
> >        * t is only needed to create the thread; once qemu_thread_create
> >        * returns, it can safely be discarded.
> >        */
> >       QemuThread t;
> >   
> > -    if (!kvm_check_extension(kvm_state, 
> > KVM_CAP_S390_PROTECTED_ASYNC_DISABLE)) {
> > +    /* Avoid the overhead of asynchronous teardown for small machines */
> > +    if ((machine->maxram_size < 4 * GiB) ||
> > +        !kvm_check_extension(kvm_state, 
> > KVM_CAP_S390_PROTECTED_ASYNC_DISABLE)) {
> >           return false;
> >       }
> >       if (s390_pv_cmd(KVM_PV_ASYNC_CLEANUP_PREPARE, NULL) != 0) {  
> 
>   Thomas
>