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

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi Marc,

> On Wed, 25 Feb 2026 18:27:07 +0000,
> Yeoreum Yun <yeoreum.yun@arm.com> 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@arm.com>
> > ---
> >  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.

Okay. But as Suzuki mention, I think it seems to be redundant.
I'll remove it.

>
> > 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.

Okay. I'll fold it into #5.

>
> > 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.

That was my origin thought but there was relevant discussion about this:
  - https://lore.kernel.org/all/aW5dzb0ldp8u8Rdm@willie-the-truck/
  - https://lore.kernel.org/all/aYtZfpWjRJ1r23nw@arm.com/

In summary, I couldn't make that assumption --
PAN always presents when LSUI presents for :

   - CPU bugs happen all the time
   - Virtualisation and idreg overrides mean illegal feature combinations
    can show up

So, uaccess_ttbr0_enable() is for when SW_PAN is enabled.

I'll make a comment for this.

[...]

Thanks!

--
Sincerely,
Yeoreum Yun