linux-kernel - Re: [PATCH v3 2/2] arch/powerpc/kvm: kvmppc_hv

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Date:   Tue, 21 Mar 2023 14:15:14 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Kautuk Consul" <kconsul@...ux.vnet.ibm.com>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Fabiano Rosas" <farosas@...ux.ibm.com>,
        "Sathvika Vasireddy" <sv@...ux.ibm.com>,
        "Alexey Kardashevskiy" <aik@...abs.ru>
Cc:     <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] arch/powerpc/kvm: kvmppc_hv_entry: remove r4
 argument

On Thu Mar 16, 2023 at 3:10 PM AEST, Kautuk Consul wrote:
> kvmppc_hv_entry is called from only 2 locations within
> book3s_hv_rmhandlers.S. Both of those locations set r4
> as HSTATE_KVM_VCPU(r13) before calling kvmppc_hv_entry.
> So, shift the r4 load instruction to kvmppc_hv_entry and
> thus modify the calling convention of this function.

Is r4 there only used for CONFIG_KVM_BOOK3S_HV_P8_TIMING? Could put it
under there. Although you then lose the barrier if it's disabled, that
is okay if you're sure that's the only memory operation being ordered.

I'm not sure how much new work we want to put into changing this asm
code, since it's POWER7/8 only. I would love to move this (and the
other) KVM implementations to C like we did with P9. It's a pretty big
job though.

Thanks,
Nick

>
> Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_hv_rmhandlers.S | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index b81ba4ee0521..b61f0b2c677b 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -85,7 +85,7 @@ _GLOBAL_TOC(kvmppc_hv_entry_trampoline)
>  	RFI_TO_KERNEL
>  
>  kvmppc_call_hv_entry:
> -	ld	r4, HSTATE_KVM_VCPU(r13)
> +	/* Enter guest. */
>  	bl	kvmppc_hv_entry
>  
>  	/* Back from guest - restore host state and return to caller */
> @@ -352,9 +352,7 @@ kvm_secondary_got_guest:
>  	mtspr	SPRN_LDBAR, r0
>  	isync
>  63:
> -	/* Order load of vcpu after load of vcore */
> -	lwsync
> -	ld	r4, HSTATE_KVM_VCPU(r13)
> +	/* Enter guest. */
>  	bl	kvmppc_hv_entry
>  
>  	/* Back from the guest, go back to nap */
> @@ -506,7 +504,6 @@ SYM_INNER_LABEL(kvmppc_hv_entry, SYM_L_LOCAL)
>  
>  	/* Required state:
>  	 *
> -	 * R4 = vcpu pointer (or NULL)
>  	 * MSR = ~IR|DR
>  	 * R13 = PACA
>  	 * R1 = host R1
> @@ -524,6 +521,10 @@ SYM_INNER_LABEL(kvmppc_hv_entry, SYM_L_LOCAL)
>  	li	r6, KVM_GUEST_MODE_HOST_HV
>  	stb	r6, HSTATE_IN_GUEST(r13)
>  
> +	/* Order load of vcpu after load of vcore */
> +	lwsync
> +	ld	r4, HSTATE_KVM_VCPU(r13)
> +
>  #ifdef CONFIG_KVM_BOOK3S_HV_P8_TIMING
>  	/* Store initial timestamp */
>  	cmpdi	r4, 0
> -- 
> 2.39.2