Re: [PATCH v2 4/7] x86/xen: support nested lazy_mmu sections (again)

David Hildenbrand
2025-09-09T09:14:03Z

On 08.09.25 09:39, Kevin Brodsky wrote:
> Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode")
> originally introduced support for nested lazy sections (LAZY_MMU and
> LAZY_CPU). It later got reverted by commit c36549ff8d84 as its
> implementation turned out to be intolerant to preemption.
>
> Now that the lazy_mmu API allows enter() to pass through a state to
> the matching leave() call, we can support nesting again for the
> LAZY_MMU mode in a preemption-safe manner. If xen_enter_lazy_mmu() is
> called inside an active lazy_mmu section, xen_lazy_mode will already
> be set to XEN_LAZY_MMU and we can then return LAZY_MMU_NESTED to
> instruct the matching xen_leave_lazy_mmu() call to leave
> xen_lazy_mode unchanged.
>
> The only effect of this patch is to ensure that xen_lazy_mode
> remains set to XEN_LAZY_MMU until the outermost lazy_mmu section
> ends. xen_leave_lazy_mmu() still calls xen_mc_flush()
> unconditionally.
>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
> arch/x86/include/asm/paravirt.h | 6 ++----
> arch/x86/include/asm/paravirt_types.h | 4 ++--
> arch/x86/xen/mmu_pv.c | 11 ++++++++---
> 3 files changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 65a0d394fba1..4ecd3a6b1dea 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -529,14 +529,12 @@ static inline void arch_end_context_switch(struct task_struct *next)
> #define __HAVE_ARCH_ENTER_LAZY_MMU_MODE
> static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void)
> {
> - PVOP_VCALL0(mmu.lazy_mode.enter);
> -
> - return LAZY_MMU_DEFAULT;
> + return PVOP_CALL0(lazy_mmu_state_t, mmu.lazy_mode.enter);
> }
>
> static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
> {
> - PVOP_VCALL0(mmu.lazy_mode.leave);
> + PVOP_VCALL1(mmu.lazy_mode.leave, state);
> }
>
> static inline void arch_flush_lazy_mmu_mode(void)
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index bc1af86868a3..b7c567ccbf32 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -45,8 +45,8 @@ typedef int lazy_mmu_state_t;
>
> struct pv_lazy_ops {
> /* Set deferred update mode, used for batching operations. */
> - void (*enter)(void);
> - void (*leave)(void);
> + lazy_mmu_state_t (*enter)(void);
> + void (*leave)(lazy_mmu_state_t);
> void (*flush)(void);
> } __no_randomize_layout;
> #endif
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 2039d5132ca3..6e5390ff06a5 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -2130,9 +2130,13 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
> #endif
> }
>
> -static void xen_enter_lazy_mmu(void)
> +static lazy_mmu_state_t xen_enter_lazy_mmu(void)
> {
> + if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
> + return LAZY_MMU_NESTED;
> +

You mention above "preemption-safe manner" above, so I am wondering,
what if we get preempted immediately after doing the this_cpu_read() and
get scheduled on another CPU?

--
Cheers

David / dhildenb