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

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 09/09/2025 11:56, David Hildenbrand wrote:
> 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. 

That's a fair point, Alexander was also wondering in v1 (and so was I
when I worked on this patch). Will clarify in v3.

- Kevin