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Subject: Re: [PATCH 2/3] KVM: PPC: Book3S HV: Signed extend decrementer value
 if not using large decr
To: Suraj Jitindar Singh <sjitindarsingh@gmail.com>,
 linuxppc-dev@lists.ozlabs.org
References: <20190620014651.7645-1-sjitindarsingh@gmail.com>
 <20190620014651.7645-2-sjitindarsingh@gmail.com>
From: Laurent Vivier <lvivier@redhat.com>
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Date: Thu, 20 Jun 2019 09:56:51 +0200
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On 20/06/2019 03:46, Suraj Jitindar Singh wrote:
> On POWER9 the decrementer can operate in large decrementer mode where
> the decrementer is 56 bits and signed extended to 64 bits. When not
> operating in this mode the decrementer behaves as a 32 bit decrementer
> which is NOT signed extended (as on POWER8).
> 
> Currently when reading a guest decrementer value we don't take into
> account whether the large decrementer is enabled or not, and this means
> the value will be incorrect when the guest is not using the large
> decrementer. Fix this by sign extending the value read when the guest
> isn't using the large decrementer.
> 
> Fixes: 95a6432ce903 "KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests"
> 
> Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index d3684509da35..719fd2529eec 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3607,6 +3607,8 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
>  
>  	vcpu->arch.slb_max = 0;
>  	dec = mfspr(SPRN_DEC);
> +	if (!(lpcr & LPCR_LD)) /* Sign extend if not using large decrementer */
> +		dec = (s32) dec;
>  	tb = mftb();
>  	vcpu->arch.dec_expires = dec + tb;
>  	vcpu->cpu = -1;
> 

Patches 2 and 3: tested I can boot and run an L2 nested guest with qemu
v4.0.0 and caps-large-decr=on in the case we have had a hang previously.

Tested-by: Laurent Vivier <lvivier@redhat.com>