linux-kernel - Re: [PATCH] KVM: arm64: Use pointer from memcpy() call for assignment in init_hyp

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Message-ID: <20251030212151.352e9a8a@pumpkin>
Date: Thu, 30 Oct 2025 21:21:51 +0000
From: David Laight <david.laight.linux@...il.com>
To: Mark Rutland <mark.rutland@....com>
Cc: Markus Elfring <Markus.Elfring@....de>, kvmarm@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org, Catalin Marinas
 <catalin.marinas@....com>, David Brazdil <dbrazdil@...gle.com>, Joey Gouly
 <joey.gouly@....com>, Marc Zyngier <maz@...nel.org>, Oliver Upton
 <oliver.upton@...ux.dev>, Suzuki Poulouse <suzuki.poulose@....com>, Will
 Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>, LKML
 <linux-kernel@...r.kernel.org>, kernel-janitors@...r.kernel.org, Miaoqian
 Lin <linmq006@...il.com>
Subject: Re: [PATCH] KVM: arm64: Use pointer from memcpy() call for
 assignment in init_hyp_mode()

On Thu, 30 Oct 2025 17:49:51 +0000
Mark Rutland <mark.rutland@....com> wrote:

> On Thu, Oct 30, 2025 at 06:11:03PM +0100, Markus Elfring wrote:
> > From: Markus Elfring <elfring@...rs.sourceforge.net>
> > Date: Thu, 30 Oct 2025 18:01:41 +0100
> > 
> > A pointer was assigned to a variable. The same pointer was used for
> > the destination parameter of a memcpy() call.
> > This function is documented in the way that the same value is returned.
> > Thus convert two separate statements into a direct variable assignment for
> > the return value from a memory copy action.
> > 
> > The source code was transformed by using the Coccinelle software.
> > 
> > Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> > ---
> >  arch/arm64/kvm/arm.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 870953b4a8a7..feab88c31703 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -2600,8 +2600,8 @@ static int __init init_hyp_mode(void)
> >  			goto out_err;
> >  		}
> >  
> > -		page_addr = page_address(page);
> > -		memcpy(page_addr, CHOOSE_NVHE_SYM(__per_cpu_start), nvhe_percpu_size());
> > +		page_addr = memcpy(page_address(page), CHOOSE_NVHE_SYM(__per_cpu_start),
> > +				   nvhe_percpu_size());  
> 
> This change makes the code harder to read, and harder to modify. It
> saves no space.

It might save a register spill - but really isn't worth the effort.
memcpy() is really best treated as being 'void'.
Indeed most implementations would be better if it were 'void'.
Although you could define:
#define memcpy(d, s, l) ({ auto _d = d; void_memcpy(_d, s, l); _d})
so that the compiler would optimise away the save that memcpy() typically
has to do.

I even suspect that memcpy() is an old enough function that the return
value is 'what the implementation happened to leave in r0'.

	David


> 
> As Dan said [1]:
> 
> | No one will thank you for making these changes...  :(  Please don't do
> | it.
> 
> [1] https://lore.kernel.org/lkml/aQNsecHJSO2U68Fc@stanley.mountain/
> 
> Mark.
>