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

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Date:   Thu, 16 Mar 2023 09:19:25 +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

Hi,

On 2023-03-16 14:39:08, Michael Ellerman wrote:
> Kautuk Consul <kconsul@...ux.vnet.ibm.com> writes:
> > 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 ?
> 
> No.
> 
> > Or do you mean to say that pending loads may not be
> > cleared/flushed across the "bl <funcname>" boundary ?
> 
> Right.
> 
> The "bl" imposes no ordering on loads before or after it.
> 
> In general nothing orders two independant loads, other than a barrier.
> 
> cheers

Okay, I will post a patch v3 with lwsync before the load to r4 in
kvmppc_hv_entry.

Thanks.