Re: [PATCH RFC 2/3] x86/mm: implement redundant IPI elimination for

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/22/25 04:19, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> 
> On Thu, 18 Dec 2025 14:08:07 +0100, David Hildenbrand (Red Hat) wrote:
>> On 12/13/25 09:00, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@linux.dev>
>>>
>>> Pass both freed_tables and unshared_tables to flush_tlb_mm_range() to
>>> ensure lazy-TLB CPUs receive IPIs and flush their paging-structure caches:
>>>
>>> 	flush_tlb_mm_range(..., freed_tables || unshared_tables);
>>>
>>> Implement tlb_table_flush_implies_ipi_broadcast() for x86: on native x86
>>> without paravirt or INVLPGB, the TLB flush IPI already provides necessary
>>> synchronization, allowing the second IPI to be skipped. For paravirt with
>>> non-native flush_tlb_multi and for INVLPGB, conservatively keep both IPIs.
>>>
>>> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>>> ---
>>>    arch/x86/include/asm/tlb.h | 17 ++++++++++++++++-
>>>    1 file changed, 16 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
>>> index 866ea78ba156..96602b7b7210 100644
>>> --- a/arch/x86/include/asm/tlb.h
>>> +++ b/arch/x86/include/asm/tlb.h
>>> @@ -5,10 +5,24 @@
>>>    #define tlb_flush tlb_flush
>>>    static inline void tlb_flush(struct mmu_gather *tlb);
>>>    
>>> +#define tlb_table_flush_implies_ipi_broadcast tlb_table_flush_implies_ipi_broadcast
>>> +static inline bool tlb_table_flush_implies_ipi_broadcast(void);
>>> +
>>>    #include <asm-generic/tlb.h>
>>>    #include <linux/kernel.h>
>>>    #include <vdso/bits.h>
>>>    #include <vdso/page.h>
>>> +#include <asm/paravirt.h>
>>> +
>>> +static inline bool tlb_table_flush_implies_ipi_broadcast(void)
>>> +{
>>> +#ifdef CONFIG_PARAVIRT
>>> +	/* Paravirt may use hypercalls that don't send real IPIs. */
>>> +	if (pv_ops.mmu.flush_tlb_multi != native_flush_tlb_multi)
>>> +		return false;
>>> +#endif
>>> +	return !cpu_feature_enabled(X86_FEATURE_INVLPGB);
>>
>> Right, here I was wondering whether we should have a new pv_ops callback
>> to indicate that instead.
>>
>> pv_ops.mmu.tlb_table_flush_implies_ipi_broadcast()
>>
>> Or a simple boolean property that pv init code properly sets.
> 
> Cool!
> 
>>
>> Something for x86 folks to give suggestions for. :)
> 
> I prefer to use a boolean property instead of comparing function pointers.
> Something like this:
> 
> ----8<----
> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
> index cfcb60468b01..90e9da33f2c7 100644
> --- a/arch/x86/hyperv/mmu.c
> +++ b/arch/x86/hyperv/mmu.c
> @@ -243,4 +243,5 @@ void hyperv_setup_mmu_ops(void)
> 
>   	pr_info("Using hypercall for remote TLB flush\n");
>   	pv_ops.mmu.flush_tlb_multi = hyperv_flush_tlb_multi;
> +	pv_ops.mmu.tlb_flush_implies_ipi_broadcast = false;
>   }
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index 3502939415ad..f9756df6f3f6 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -133,6 +133,19 @@ struct pv_mmu_ops {
>   	void (*flush_tlb_multi)(const struct cpumask *cpus,
>   				const struct flush_tlb_info *info);
> 
> +	/*
> +	 * Indicates whether TLB flush IPIs provide sufficient synchronization
> +	 * for GUP-fast when freeing or unsharing page tables.
> +	 *
> +	 * Set to true only when the TLB flush guarantees:
> +	 * - IPIs reach all CPUs with potentially stale paging-structure caches
> +	 * - Synchronization with IRQ-disabled code like GUP-fast
> +	 *
> +	 * Paravirt implementations that use hypercalls (which may not send
> +	 * real IPIs) should set this to false.
> +	 */
> +	bool tlb_flush_implies_ipi_broadcast;
> +
>   	/* Hook for intercepting the destruction of an mm_struct. */
>   	void (*exit_mmap)(struct mm_struct *mm);
>   	void (*notify_page_enc_status_changed)(unsigned long pfn, int npages, bool enc);
> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
> index 96602b7b7210..9d20ad4786cc 100644
> --- a/arch/x86/include/asm/tlb.h
> +++ b/arch/x86/include/asm/tlb.h
> @@ -18,7 +18,7 @@ static inline bool tlb_table_flush_implies_ipi_broadcast(void)
>   {
>   #ifdef CONFIG_PARAVIRT
>   	/* Paravirt may use hypercalls that don't send real IPIs. */
> -	if (pv_ops.mmu.flush_tlb_multi != native_flush_tlb_multi)
> +	if (!pv_ops.mmu.tlb_flush_implies_ipi_broadcast)
>   		return false;
>   #endif
>   	return !cpu_feature_enabled(X86_FEATURE_INVLPGB);

I'd have thought that the X86_FEATURE_INVLPGB heck should then also be 
taken care of by whoever sets tlb_flush_implies_ipi_broadcast.

-- 
Cheers

David