Re: [PATCH v5 5/5] target/ppc: Fix regression due to Power10 and Power11 having same PCR

[Aditya Gupta]

On 23/07/24 10:28, Nicholas Piggin wrote:
> On Thu Jun 6, 2024 at 10:16 PM AEST, Aditya Gupta wrote:
> > Power11 has the same PCR (Processor Compatibility Register) value, as
> > Power10.
> >
> > Due to this, QEMU considers Power11 as a valid compat-mode for Power10,
> > ie. earlier it was possible to run QEMU with '-M 
> > pseries,max-compat-mode=power11 --cpu power10'
> Isn't this expected to work, or no?

It works, but it didn't feel logical to be able to boot an older CPU in 
a compat mode of a newer CPU.

Major reason though, it caused regression where `-M pseries --cpu 
power10` was booting as Power11, since Power11 was the highest PCR it 
found as compatible.

The only issue I see in this patch is the assumption that a newer 
processor must always have a higher PVR value, which is true as of now.


> > Same PCR also introduced a regression where `-M pseries --cpu power10`
> > boots as Power11 (ie. logical PVR is of Power11, even though PVR is Power10).
> > The regression was due to 'do_client_architecture_support' checking for
> > valid compat modes and finding Power11 to be a valid compat mode for
> > Power10 (it happens even without passing 'max-compat-mode' explicitly).
> >
> > Fix compat-mode issue and regression, by ensuring a future Power
> > processor (with a higher logical_pvr value, eg. P11) cannot be valid
> > compat-mode for an older Power processor (eg. P10)
> This should be done before introducing the Power11 CPU so there's no
> regression inside the series.

Sure, I will move it before the 'Add Power11 DD2.0 processor' patch.

> > Cc: Cédric Le Goater <clg@kaod.org>
> > Cc: Daniel Henrique Barboza <danielhb413@gmail.com>
> > Cc: Harsh Prateek Bora <harshpb@linux.ibm.com>
> > Cc: Mahesh J Salgaonkar <mahesh@linux.ibm.com>
> > Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
> > ---
> >   target/ppc/compat.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/target/ppc/compat.c b/target/ppc/compat.c
> > index 12dd8ae290ca..168a3c06316f 100644
> > --- a/target/ppc/compat.c
> > +++ b/target/ppc/compat.c
> > @@ -139,6 +139,10 @@ static bool pcc_compat(PowerPCCPUClass *pcc, uint32_t 
> > compat_pvr,
> >           /* Outside specified range */
> >           return false;
> >       }
> > +    if (compat->pvr > pcc->logical_pvr) {
> > +        /* Older CPU cannot support a newer processor's compat mode */
> > +        return false;
> > +    }
> Hmm. I suppose this is the right way to fix it.
>
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

Thank you for the tag, Nick !


- Aditya Gupta

> >       if (!(pcc->pcr_supported & compat->pcr_level)) {
> >           /* Not supported by this CPU */
> >           return false;