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

[Nicholas Piggin]

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?

>
> 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.

>
> 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>

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