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Re: [PATCH 2/5] s390x/pv: Implement CGS check handler


From: Thomas Huth
Subject: Re: [PATCH 2/5] s390x/pv: Implement CGS check handler
Date: Thu, 5 Jan 2023 12:42:54 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0

On 04/01/2023 12.51, Cédric Le Goater wrote:
From: Cédric Le Goater <clg@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 <clg@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);

  Thomas




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