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

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Date:   Tue, 21 Mar 2023 10:24:28 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     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>,
        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

Hi,

On 2023-03-21 14:15:14, Nicholas Piggin wrote:
> 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.
r4 is also used by the secondary_too_late label. So I decided against
moving it inside the CONFIG_KVM_BOOK3S_HV_P8_TIMING #ifdef as then I
would need to again load from HSTATE_KVM_VCPU(r13) in secondary_too_late.
> 
> 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.
Yeah. I was just reviewing this code and decided to send this small
improvement to the mailing list. I will check with my team and ask
them if we could move this implementation to C.
> 
> 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
>