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

[Thomas Huth]

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

> 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.

>       /*
>        * 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