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

Jürgen Groß
2025-09-09T09:37:41Z

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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.

Juergen

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