On Thu, 5 Jan 2023 12:42:54 +0100
Thomas Huth <th...@redhat.com> wrote:
> On 04/01/2023 12.51, Cédric Le Goater wrote:
> > From: Cédric Le Goater <c...@redhat.com>
> >
> > When a protected VM is started with the maximum number of CPUs (248),
> > the service call providing information on the CPUs requires more
> > buffer space than allocated and QEMU disgracefully aborts :
> >
> > LOADPARM=[........]
> > Using virtio-blk.
> > Using SCSI scheme.
> >
> > ...................................................................................
> > qemu-system-s390x: KVM_S390_MEM_OP failed: Argument list too long
> >
> > Implement a test for this limitation in the ConfidentialGuestSupportClass
> > check handler and provide some valid information to the user before the
> > machine starts.
> >
> > Signed-off-by: Cédric Le Goater <c...@redhat.com>
> > ---
> > hw/s390x/pv.c | 23 +++++++++++++++++++++++
> > 1 file changed, 23 insertions(+)
> >
> > diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
> > index 8dfe92d8df..3a7ec70634 100644
> > --- a/hw/s390x/pv.c
> > +++ b/hw/s390x/pv.c
> > @@ -266,6 +266,26 @@ int s390_pv_kvm_init(ConfidentialGuestSupport *cgs,
> > Error **errp)
> > return 0;
> > }
> >
> > +static bool s390_pv_check_cpus(Error **errp)
> > +{
> > + MachineState *ms = MACHINE(qdev_get_machine());
> > + MachineClass *mc = MACHINE_GET_CLASS(ms);
> > + uint32_t pv_max_cpus = mc->max_cpus - 1;
>
> Not sure whether "mc->max_cpus - 1" is the right approach here. I think it
> would be better to calculate the amount of CPUs that we can support.
>
> So AFAIK the problem is that SCLP information that is gathered during
> read_SCP_info() in hw/s390x/sclp.c. If protected virtualization is enabled,
> everything has to fit in one page (i.e. 4096 bytes) there.
>
> So we have space for
>
> (TARGET_PAGE_SIZE - offset_cpu) / sizeof(CPUEntry)
>
> CPUs.
>
> With S390_FEAT_EXTENDED_LENGTH_SCCB enabled, offset_cpu is 144 (see struct
> ReadInfo in sclp.h), otherwise it is 128.
>
> That means, with S390_FEAT_EXTENDED_LENGTH_SCCB we can have a maximum of:
>
> (4096 - 144) / 16 = 247 CPUs
>
> which is what you were trying to check with the mc->max_cpus - 1 here.
>
> But with "-cpu els=off", it sounds like we could fit all 248 also with
> protected VMs? Could you please give it a try?
>
> Anyway, instead of using "pv_max_cpus = mc->max_cpus - 1" I'd suggest to use
> something like this instead:
>
> int offset_cpu = s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) ?
> offsetof(ReadInfo, entries) :
> SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET;
> pv_max_cpus = (TARGET_PAGE_SIZE - offset_cpu) /sizeof(CPUEntry);
I agree with Thomas here
>
> Thomas
>
>
