linux-kernel - Re: [PATCH v2 2/2] arch/powerpc/kvm: kvmppc_hv

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Message-ID: <ZBFVAxAFsUJtuzEy@li-a450e7cc-27df-11b2-a85c-b5a9ac31e8ef.ibm.com>
Date:   Wed, 15 Mar 2023 10:47:55 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] arch/powerpc/kvm: kvmppc_hv_entry: remove r4
 argument

On 2023-03-15 15:48:53, Michael Ellerman wrote:
> Kautuk Consul <kconsul@...ux.vnet.ibm.com> writes:
> > 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.
> >
> > Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kvm/book3s_hv_rmhandlers.S | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> > index b81ba4ee0521..da9a15db12fe 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
> 
> Where did this barrier go?
> 
> I don't see that it's covered by any existing barriers in
> kvmppc_hv_entry, and you don't add it back anywhere. 

My concept about this is that since now the call to kvmppc_hv_entry
is first taken before the load to r4 shouldn't the pending load in the
pipeline of the HSTATE_KVM_VCORE as per the earlier comment be ordered anyway
before-hand ? Or do you mesn to say that pending loads may not be
cleared/flushed across the "bl <funcname>" boundary ?
> 
> > -	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 +521,8 @@ SYM_INNER_LABEL(kvmppc_hv_entry, SYM_L_LOCAL)
> >  	li	r6, KVM_GUEST_MODE_HOST_HV
> >  	stb	r6, HSTATE_IN_GUEST(r13)
> >  
> > +	ld	r4, HSTATE_KVM_VCPU(r13)
> > +
> >  #ifdef CONFIG_KVM_BOOK3S_HV_P8_TIMING
> >  	/* Store initial timestamp */
> >  	cmpdi	r4, 0
> 
> cheers