On Wed, 25 Feb 2026 18:27:07 +0000,
Yeoreum Yun <[email protected]> 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 <[email protected]>
> ---
> 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;
It would make more sense to move this hunk to the first patch, where
you deal with features and capabilities, instead of having this in a
random KVM-specific patch.
> 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 */
Similarly, fold this into the patch that introduces FEAT_LSUI support
for futexes (#5) so that the code is in its final position from the
beginning. This will avoid churn that makes the patches pointlessly
hard to follow, since this change is unrelated to KVM.
> 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();
Why do we need this? If FEAT_LSUI is present, than FEAT_PAN is also
present. And since PAN support not a compilation option anymore, we
should be able to rely on PAN being enabled.
Or am I missing something? If so, please document why we require it.
> +
> + 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))
> + r = __lsui_swap_desc(ptep, old, new);
> + else if (cpus_have_final_cap(ARM64_HAS_LSE_ATOMICS))
> r = __lse_swap_desc(ptep, old, new);
> else
> r = __llsc_swap_desc(ptep, old, new);
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
