linux-kernel - Re: [PATCH 1/5] KVM: selftests: Implement memcmp(), memcpy(), and memset() for guest use

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Date:   Thu, 22 Sep 2022 17:40:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Jones <andrew.jones@...ux.dev>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH 1/5] KVM: selftests: Implement memcmp(), memcpy(), and
 memset() for guest use

On Thu, Sep 22, 2022, David Matlack wrote:
> > +LIBKVM_STRING += lib/kvm_string.c
> 
> Can this file be named lib/string.c instead? This file has nothing to do
> with KVM per-se.

Yes and no.  I deliberately chose kvm_string to avoid confusion with
tools/lib/string.c and tools/include/nolibc/string.h.  The implementations
themselves aren't KVM specific, but the reason the file _exists_ is 100% unique
to KVM as there is no other environment where tools and/or selftests link to
glibc but need to override the string ops.

I'm not completely opposed to calling it string.c, but my preference is to keep
it kvm_string.c so that it's slightly more obvious that KVM selftests are a
special snowflake.

> > diff --git a/tools/testing/selftests/kvm/lib/kvm_string.c b/tools/testing/selftests/kvm/lib/kvm_string.c
> > new file mode 100644
> > index 000000000000..a60d56d4e5b8
> > --- /dev/null
> > +++ b/tools/testing/selftests/kvm/lib/kvm_string.c
> > @@ -0,0 +1,33 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +#include "kvm_util.h"
> 
> Is this include necesary?

Nope, I added the include because I also added declarations in kvm_util_base.h,
but that's unnecessary because stddef.h also provides the declarations, and those
_must_ match the prototypes of the definitions.  So yeah, this is better written as:

// SPDX-License-Identifier: GPL-2.0-only
#include <stddef.h>

/*
 * Override the "basic" built-in string helpers so that they can be used in
 * guest code.  KVM selftests don't support dynamic loading in guest code and
 * will jump into the weeds if the compiler decides to insert an out-of-line
 * call via the PLT.
 */
int memcmp(const void *cs, const void *ct, size_t count)
{
	const unsigned char *su1, *su2;
	int res = 0;

	for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--) {
		if ((res = *su1 - *su2) != 0)
			break;
	}
	return res;
}

void *memcpy(void *dest, const void *src, size_t count)
{
	char *tmp = dest;
	const char *s = src;

	while (count--)
		*tmp++ = *s++;
	return dest;
}

void *memset(void *s, int c, size_t count)
{
	char *xs = s;

	while (count--)
		*xs++ = c;
	return s;
}