Re: [Qemu-devel] [PATCH] target/ppc: Only set PCR in kvm if actually in a compat mode

[David Gibson]

On Thu, Jun 29, 2017 at 02:59:39PM +1000, Suraj Jitindar Singh wrote:
> The Processor Compatibility Register (PCR) I used to set the
> compatibility mode of the processor using the SET_ONE_REG ioctl on
> KVM_REG_PPC_ARCH_COMPAT. Previously this was only called when a compat
> mode was actually in use, however a recent patch made it unconditional.
> Calling this in KVM_PR fails as there is no handler for that call and it
> is thus impossible to start a machine with KVM_PR.
> 
> Change ppc_set_compat() so that the ioctl is only actually called if a
> compat mode is in use. This means that a KVM_PR guest can boot.
> Additionally the current behaviour for KVM_HV is preserved where a compat
> mode of 0 set pcr and arch_compat in the vcore struct to zero, both of
> which are initialised to zero anyway.
> 
> Fixes: 37f516defa2e ("pseries: Reset CPU compatibility mode")
> 
> Signed-off-by: Suraj Jitindar Singh <address@hidden>

This doesn't seem quite right.  With this change, how would we ever
turn compatibility mode _off_ (which could happen on reset if nothing
else).  Really we should add this pseudo-register to KVM PR, although
I'm fine with also having a qemu workaround to let it work with older
PR versions.

> 
> ---
> 
> Based on: dwg/ppc-for-2.10
> 
>  target/ppc/compat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/ppc/compat.c b/target/ppc/compat.c
> index f1b67fa..4482206 100644
> --- a/target/ppc/compat.c
> +++ b/target/ppc/compat.c
> @@ -143,7 +143,7 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, 
> Error **errp)
>      cpu->compat_pvr = compat_pvr;
>      env->spr[SPR_PCR] = pcr & pcc->pcr_mask;
>  
> -    if (kvm_enabled()) {
> +    if (kvm_enabled() && compat_pvr) {
>          int ret = kvmppc_set_compat(cpu, cpu->compat_pvr);
>          if (ret < 0) {
>              error_setg_errno(errp, -ret,

-- 
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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