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

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Date:   Thu, 23 Mar 2023 10:23:59 +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>,
        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 2023-03-22 23:17:35, 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 | 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 */
> 
> Just copying the comment here doesn't work. It doesn't make sense on its
> own here, because the VCORE is loaded (again) a few lines below (536).
> So as written this comment seems backward vs the code.
> 
> The comment would need to expand to explain that the barrier is for the
> case where we came from kvm_secondary_got_guest.
> 
Agreed.
> > +	lwsync
> > +	ld	r4, HSTATE_KVM_VCPU(r13)
> > +
> >  #ifdef CONFIG_KVM_BOOK3S_HV_P8_TIMING
> >  	/* Store initial timestamp */
> >  	cmpdi	r4, 0
> 
> 
> But as Nick says I don't think it's worth investing effort in small
> tweaks to this code. The risk of introducing bugs is too high for such a
> small improvement to the code.
> 
> Thanks for trying, but I think this asm code is best left more or less
> alone unless we find actual bugs in it - or unless we can make
> substantial improvements to it, which would be rewriting in C, or at
> least converting to a fully call/return style rather than the current
> forest of labels.
> 
> I will take patch 1 though, as that's an obvious cleanup and poses no
> risk (famous last words :D).
Thanks for taking patch 1. I completely agree that it would not be wise
to introduce even small alterations to stable legacy code. But sending
patches like this and conversing with this mailing list's reviewers
is giving me a good picture of what you are open to accepting at this
stage.

Thanks again!
> 
> cheers