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

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Date:   Thu, 16 Mar 2023 14:40:38 +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

Michael Ellerman <mpe@...erman.id.au> writes:
> 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.

There's some docs on barriers here:

  https://www.kernel.org/doc/Documentation/memory-barriers.txt

Though admittedly it is pretty dense.

cheers