Re: [Qemu-devel] [PATCH] target/ppc: Allow workarounds for POWER9 DD1

[Thomas Huth <thuth@redhat.com>]

On 09.05.2017 05:45, David Gibson wrote:
> POWER9 DD1 silicon has some bugs which mean it a) isn't really compliant
> with the ISA v3.00 and b) require a number of special workarounds in the
> kernel.
> 
> At the moment, qemu isn't aware of DD1.  For TCG we don't really want it to
> be (why bother emulating buggy silicon).  But with KVM, the guest does need
> to be aware of DD1 so it can apply the necessary workarounds.
> 
> Meanwhile, the feature negotiation between qemu and the guest strongly
> favours architected compatibility modes to "raw" CPU modes.  In combination
> with the above, this means the guest sees architected POWER9 mode, and
> doesn't apply the DD1 workarounds.  Well, unless it has yet another
> workaround to partially ignore what qemu tells it.
> 
> This patch addresses this by disabling support for compatibility modes when
> using KVM on a POWER9 DD1 host.

I first though: Hey, it should be fixed in the guest kernel instead, but
thinking about this twice, I think you're right. If the CPU is not fully
compatible to the ISA, we really should not announce it as "architected
/ compatible POWER9" in QEMU.
So basically ACK to your patch, I've just got a cosmetic request below...

> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  target/ppc/kvm.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 8574c36..591b5b5 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2380,6 +2380,17 @@ static void kvmppc_host_cpu_class_init(ObjectClass *oc, void *data)
>  
>  #if defined(TARGET_PPC64)
>      pcc->radix_page_info = kvm_get_radix_page_info();
> +
> +    if ((pcc->pvr & 0xffffff00) == 0x004e0100) {

Could you please add a proper #define for that magic DD1.0 value to
cpu-models.h, please?

 Thanks,
  Thomas