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

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

[David Gibson]

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.

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) {
+        /*
+         * POWER9 DD1 has some bugs which make it not really ISA 3.00
+         * compliant.  More importantly, advertising ISA 3.00
+         * architected mode may prevent guests from activating
+         * necessary DD1 workarounds.
+         */
+        pcc->pcr_supported &= ~(PCR_COMPAT_3_00 | PCR_COMPAT_2_07
+                                | PCR_COMPAT_2_06 | PCR_COMPAT_2_05);
+    }
 #endif /* defined(TARGET_PPC64) */
 }
 
-- 
2.9.3

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

[Thomas Huth]

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

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

[David Gibson]

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On Tue, May 09, 2017 at 06:41:23AM +0200, Thomas Huth wrote:
> 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?

Good point, done.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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