Subject: Re: [PATCH] powerpc/kvm: Fix kvmppc_vcore->in_guest value in kvmhv_switch_to_host

From: Alistair Popple <[email-masked]>
Date: Tue, 08 Oct 2019 10:25:30 +1100

On Friday, 4 October 2019 12:53:17 PM AEDT Jordan Niethe wrote:
> kvmhv_switch_to_host() in arch/powerpc/kvm/book3s_hv_rmhandlers.S needs
> to set kvmppc_vcore->in_guest to 0 to signal secondary CPUs to continue.
> This happens after resetting the PCR. Before commit 13c7bb3c57dc
> ("powerpc/64s: Set reserved PCR bits"), r0 would always be 0 before it
> was stored to kvmppc_vcore->in_guest. However because of this change in
> the commit:
> 
>         /* Reset PCR */
>         ld      r0, VCORE_PCR(r5)
> -       cmpdi   r0, 0
> +       LOAD_REG_IMMEDIATE(r6, PCR_MASK)
> +       cmpld   r0, r6
>         beq     18f
> -       li      r0, 0
> -       mtspr   SPRN_PCR, r0
> +       mtspr   SPRN_PCR, r6
>  18:
>         /* Signal secondary CPUs to continue */
>         stb     r0,VCORE_IN_GUEST(r5)

Easy to understand how that was missed :-)

Reviewed-by: Alistair Popple <[email-masked]>

> We are no longer comparing r0 against 0 and loading it with 0 if it
> contains something else. Hence when we store r0 to
> kvmppc_vcore->in_guest, it might not be 0.  This means that secondary
> CPUs will not be signalled to continue. Those CPUs get stuck and errors
> like the following are logged:
> 
>     KVM: CPU 1 seems to be stuck
>     KVM: CPU 2 seems to be stuck
>     KVM: CPU 3 seems to be stuck
>     KVM: CPU 4 seems to be stuck
>     KVM: CPU 5 seems to be stuck
>     KVM: CPU 6 seems to be stuck
>     KVM: CPU 7 seems to be stuck
> 
> This can be reproduced with:
>     $ for i in `seq 1 7` ; do chcpu -d $i ; done ;
>     $ taskset -c 0 qemu-system-ppc64 -smp 8,threads=8 \
>        -M pseries,accel=kvm,kvm-type=HV -m 1G -nographic -vga none \
>        -kernel vmlinux -initrd initrd.cpio.xz
> 
> Fix by making sure r0 is 0 before storing it to kvmppc_vcore->in_guest.
> 
> Fixes: 13c7bb3c57dc ("powerpc/64s: Set reserved PCR bits")
> Reported-by: Alexey Kardashevskiy <[email-masked]>
> Signed-off-by: Jordan Niethe <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv_rmhandlers.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index 74a9cfe84aee..faebcbb8c4db 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1921,6 +1921,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>  	mtspr	SPRN_PCR, r6
>  18:
>  	/* Signal secondary CPUs to continue */
> +	li	r0, 0
>  	stb	r0,VCORE_IN_GUEST(r5)
>  19:	lis	r8,0x7fff		/* MAX_INT@h */
>  	mtspr	SPRN_HDEC,r8
>