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

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

> On 26/02/2026 14:05, Yeoreum Yun wrote:
> > 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.
> >
>
> No, this is required and Marc is right. This hunk should be part of the
> original patch that adds the cap. What I am saying is that you don't
> need to explicitly call the cpucap_is_poissible() down, but it is
> implicitly called by cpus_have_final_cap().

Ah. my bad eyes, I miss alternative_has_cap_unlikely() calls
cpucap_is_poissible().

Thanks to point out this!


>
>
>
> > >
> > > > 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
>

--
Sincerely,
Yeoreum Yun