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

David Hildenbrand
2025-09-09T09:57:00Z

On 09.09.25 11:37, Jürgen Groß wrote:
> On 09.09.25 11:13, David Hildenbrand wrote:
>> 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?
>>
>
> This should still be correct: preemption needs a context switch to happen,
> so xen_start_context_switch() and xen_end_context_switch() are involved.
> Those are dealing with this problem by doing the right thing in the old
> and the new context.

Thanks, that makes sense. Would be valuable to add that detail to the
patch description.

--
Cheers

David / dhildenb