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Re: [Qemu-devel] [PATCH qemu] spapr/target-ppc/kvm: Only add hcall-instr

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [PATCH qemu] spapr/target-ppc/kvm: Only add hcall-instructions if KVM supports it
Date: Tue, 15 Mar 2016 21:31:48 +1100
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 03/15/2016 08:59 PM, David Gibson wrote:
On Tue, Mar 15, 2016 at 04:51:20PM +1100, Alexey Kardashevskiy wrote:
ePAPR defines "hcall-instructions" device-tree property which contains
code to call hypercalls in ePAPR paravirtualized guests. However this
property is also present for pseries guests where it does not make sense,
even though it contains dummy code which simply fails.

Instead of maintaining the property (which used to be BE only; then was
fixed to be endian-agnostic) and confusing the guest (which might think
there is ePAPR host while there is none), this simply does not
the property to the device tree if the host kernel does not implement it.

In order to tell the machine code if the host kernel supports
KVM_CAP_PPC_GET_PVINFO, this changes kvmppc_get_hypercall() to return 1
if the host kernel does not implement it (which is HV KVM case).

Signed-off-by: Alexey Kardashevskiy <address@hidden>

So the idea of only adding the property when the host kernel supplies
a suitable value seems good, but I'm a bit nervous about applying
this, because I'm not sure what case the original fallback hypercall
code was supposed to handle.

agraf, if you could enlighten us with some history that could be good.


We just got a bug report that LE guests would not boot under quite old QEMU
and we (powerkvm) wonder if it makes sense to backport endian-agnostic
hypercall code to older QEMU or it is simpler/more correct
not to have epapr-hypercall property in the tree.

  hw/ppc/spapr.c   | 9 +++++----
  target-ppc/kvm.c | 2 +-
  2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 43708a2..8130eb4 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -497,10 +497,11 @@ static void *spapr_create_fdt_skel(hwaddr initrd_base,
               * Older KVM versions with older guest kernels were broken with 
               * magic page, don't allow the guest to map it.
-            kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
-                                 sizeof(hypercall));
-            _FDT((fdt_property(fdt, "hcall-instructions", hypercall,
-                              sizeof(hypercall))));
+            if (!kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
+                                      sizeof(hypercall))) {
+                _FDT((fdt_property(fdt, "hcall-instructions", hypercall,
+                                   sizeof(hypercall))));
+            }
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 776336b..e5183db 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -2001,7 +2001,7 @@ int kvmppc_get_hypercall(CPUPPCState *env, uint8_t *buf, 
int buf_len)
      hc[2] = cpu_to_be32(0x48000008);
      hc[3] = cpu_to_be32(bswap32(0x3860ffff));

Since you're now returning a value which means the caller is supposed
to ignore the hc code, there's not much point actually populating it above.

The return value means "no KVM support is here" rather than "ignore @buf content".

And the patch should have been "RFC" I suppose :)

-    return 0;
+    return 1;

  static inline int kvmppc_enable_hcall(KVMState *s, target_ulong hcall)


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