Re: [Qemu-devel] [PATCH] ppc: fix ppc_set_compat() with KVM PR

[Greg Kurz <groug@kaod.org>]

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On Tue, 15 Aug 2017 11:40:34 +1000
Suraj Jitindar Singh <sjitindarsingh@gmail.com> wrote:

> On Mon, 2017-08-14 at 19:49 +0200, Greg Kurz wrote:
> > When running in KVM PR mode, kvmppc_set_compat() always fail because
> > the
> > current PR implementation doesn't handle KVM_REG_PPC_ARCH_COMPAT. Now
> > that
> > the machine code inconditionally calls ppc_set_compat_all() at reset
> > time
> > to restore the compat mode default value (commit 66d5c492dd3a9), it
> > is
> > impossible to start a guest with PR:
> > 
> > qemu-system-ppc64: Unable to set CPU compatibility mode in KVM:
> >  Invalid argument
> > 
> > A tentative patch [1] was recently sent by Suraj to address the
> > issue, but
> > it would prevent the compat mode to be turned off on reset. And we
> > really
> > don't want to explicitely check for KVM PR. During the patch's
> > review,
> > David suggested that we should only call the KVM ioctl() if the
> > compat
> > PVR changes. This allows at least to run with KVM PR, provided no
> > compat
> > mode is requested from the command line (which should be the case
> > when
> > running PR nested). This is what this patch does.
> > 
> > While here, we also fix the side effect where KVM would fail but we
> > would
> > change the CPU state in QEMU anyway.
> > 
> > [1] http://patchwork.ozlabs.org/patch/782039/
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>  
> 
> I meant to send a follow up but it fell on my priority list so thanks
> for this
> 

No problem :)

This is more a workaround than an actual fix since CAS will still want to
change the compat mode and fail, but at least QEMU doesn't exit right away.
I needed that to chase down another bug that breaks loadvm of a KVM PR guest.

> Looks good to me
> 
> Reviewed-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
> 

Thanks!

> > ---
> >  target/ppc/compat.c |    9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/target/ppc/compat.c b/target/ppc/compat.c
> > index f1b67faa97e3..f8729fe46d61 100644
> > --- a/target/ppc/compat.c
> > +++ b/target/ppc/compat.c
> > @@ -140,16 +140,17 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t
> > compat_pvr, Error **errp)
> >  
> >      cpu_synchronize_state(CPU(cpu));
> >  
> > -    cpu->compat_pvr = compat_pvr;
> > -    env->spr[SPR_PCR] = pcr & pcc->pcr_mask;
> > -
> > -    if (kvm_enabled()) {
> > +    if (kvm_enabled() && cpu->compat_pvr != compat_pvr) {
> >          int ret = kvmppc_set_compat(cpu, cpu->compat_pvr);
> >          if (ret < 0) {
> >              error_setg_errno(errp, -ret,
> >                               "Unable to set CPU compatibility mode
> > in KVM");
> > +            return;
> >          }
> >      }
> > +
> > +    cpu->compat_pvr = compat_pvr;
> > +    env->spr[SPR_PCR] = pcr & pcc->pcr_mask;
> >  }
> >  
> >  typedef struct {
> >   


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