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

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Date:   Thu, 16 Mar 2023 14:39:08 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Kautuk Consul <kconsul@...ux.vnet.ibm.com>
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

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