Re: [PATCH v14 7/8] KVM: arm64: use CASLT instruction for swapping guest descriptor

[Yeoreum Yun]

Hi Suzuki,

> On 25/02/2026 18:27, Yeoreum Yun wrote:
> > Use the CASLT instruction to swap the guest descriptor when FEAT_LSUI
> > is enabled, avoiding the need to clear the PAN bit.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> > ---
> >   arch/arm64/include/asm/cpucaps.h |  2 ++
> >   arch/arm64/include/asm/futex.h   | 17 +----------------
> >   arch/arm64/include/asm/lsui.h    | 27 +++++++++++++++++++++++++++
> >   arch/arm64/kvm/at.c              | 30 +++++++++++++++++++++++++++++-
> >   4 files changed, 59 insertions(+), 17 deletions(-)
> >   create mode 100644 arch/arm64/include/asm/lsui.h
> >
> > diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> > index 177c691914f8..6e3da333442e 100644
> > --- a/arch/arm64/include/asm/cpucaps.h
> > +++ b/arch/arm64/include/asm/cpucaps.h
> > @@ -71,6 +71,8 @@ cpucap_is_possible(const unsigned int cap)
> >   		return true;
> >   	case ARM64_HAS_PMUV3:
> >   		return IS_ENABLED(CONFIG_HW_PERF_EVENTS);
> > +	case ARM64_HAS_LSUI:
> > +		return IS_ENABLED(CONFIG_ARM64_LSUI);
> >   	}
> >   	return true;
> > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> > index b579e9d0964d..6779c4ad927f 100644
> > --- a/arch/arm64/include/asm/futex.h
> > +++ b/arch/arm64/include/asm/futex.h
> > @@ -7,11 +7,9 @@
> >   #include <linux/futex.h>
> >   #include <linux/uaccess.h>
> > -#include <linux/stringify.h>
> > -#include <asm/alternative.h>
> > -#include <asm/alternative-macros.h>
> >   #include <asm/errno.h>
> > +#include <asm/lsui.h>
> >   #define FUTEX_MAX_LOOPS	128 /* What's the largest number you can think of? */
> > @@ -91,8 +89,6 @@ __llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> >   #ifdef CONFIG_ARM64_LSUI
> > -#define __LSUI_PREAMBLE	".arch_extension lsui\n"
> > -
> >   #define LSUI_FUTEX_ATOMIC_OP(op, asm_op)				\
> >   static __always_inline int						\
> >   __lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> > @@ -235,17 +231,6 @@ __lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> >   {
> >   	return __lsui_cmpxchg32(uaddr, oldval, newval, oval);
> >   }
> > -
> > -#define __lsui_llsc_body(op, ...)					\
> > -({									\
> > -	alternative_has_cap_unlikely(ARM64_HAS_LSUI) ?			\
> > -		__lsui_##op(__VA_ARGS__) : __llsc_##op(__VA_ARGS__);	\
> > -})
> > -
> > -#else	/* CONFIG_ARM64_LSUI */
> > -
> > -#define __lsui_llsc_body(op, ...)	__llsc_##op(__VA_ARGS__)
> > -
> >   #endif	/* CONFIG_ARM64_LSUI */
> > diff --git a/arch/arm64/include/asm/lsui.h b/arch/arm64/include/asm/lsui.h
> > new file mode 100644
> > index 000000000000..8f0d81953eb6
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/lsui.h
> > @@ -0,0 +1,27 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef __ASM_LSUI_H
> > +#define __ASM_LSUI_H
> > +
> > +#include <linux/compiler_types.h>
> > +#include <linux/stringify.h>
> > +#include <asm/alternative.h>
> > +#include <asm/alternative-macros.h>
> > +#include <asm/cpucaps.h>
> > +
> > +#define __LSUI_PREAMBLE	".arch_extension lsui\n"
> > +
> > +#ifdef CONFIG_ARM64_LSUI
> > +
> > +#define __lsui_llsc_body(op, ...)					\
> > +({									\
> > +	alternative_has_cap_unlikely(ARM64_HAS_LSUI) ?			\
> > +		__lsui_##op(__VA_ARGS__) : __llsc_##op(__VA_ARGS__);	\
> > +})
> > +
> > +#else	/* CONFIG_ARM64_LSUI */
> > +
> > +#define __lsui_llsc_body(op, ...)	__llsc_##op(__VA_ARGS__)
> > +
> > +#endif	/* CONFIG_ARM64_LSUI */
> > +
> > +#endif	/* __ASM_LSUI_H */
> > diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
> > index 885bd5bb2f41..fd3c5749e853 100644
> > --- a/arch/arm64/kvm/at.c
> > +++ b/arch/arm64/kvm/at.c
> > @@ -9,6 +9,7 @@
> >   #include <asm/esr.h>
> >   #include <asm/kvm_hyp.h>
> >   #include <asm/kvm_mmu.h>
> > +#include <asm/lsui.h>
> >   static void fail_s1_walk(struct s1_walk_result *wr, u8 fst, bool s1ptw)
> >   {
> > @@ -1704,6 +1705,31 @@ int __kvm_find_s1_desc_level(struct kvm_vcpu *vcpu, u64 va, u64 ipa, int *level)
> >   	}
> >   }
> > +static int __lsui_swap_desc(u64 __user *ptep, u64 old, u64 new)
> > +{
> > +	u64 tmp = old;
> > +	int ret = 0;
> > +
> > +	uaccess_ttbr0_enable();
> > +
> > +	asm volatile(__LSUI_PREAMBLE
> > +		     "1: caslt	%[old], %[new], %[addr]\n"
> > +		     "2:\n"
> > +		     _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w[ret])
> > +		     : [old] "+r" (old), [addr] "+Q" (*ptep), [ret] "+r" (ret)
> > +		     : [new] "r" (new)
> > +		     : "memory");
> > +
> > +	uaccess_ttbr0_disable();
> > +
> > +	if (ret)
> > +		return ret;
> > +	if (tmp != old)
> > +		return -EAGAIN;
> > +
> > +	return ret;
> > +}
> > +
> >   static int __lse_swap_desc(u64 __user *ptep, u64 old, u64 new)
> >   {
> >   	u64 tmp = old;
> > @@ -1779,7 +1805,9 @@ int __kvm_at_swap_desc(struct kvm *kvm, gpa_t ipa, u64 old, u64 new)
> >   		return -EPERM;
> >   	ptep = (u64 __user *)hva + offset;
> > -	if (cpus_have_final_cap(ARM64_HAS_LSE_ATOMICS))
> > +	if (cpucap_is_possible(ARM64_HAS_LSUI) && cpus_have_final_cap(ARM64_HAS_LSUI))
>
> minor nit:
>
> You don't need the cpucap_is_possible() as it is already checked via
> cpus_have_final_cap()->alternative_has_cap_unlikely()

Right. It seems a little bit of redundant.
I'll remove it.

Thanks!

--
Sincerely,
Yeoreum Yun