On 2026/3/23 19:10, David Hildenbrand (Arm) wrote:
On 3/9/26 03:07, Lance Yang wrote:
From: Lance Yang <[email protected]>
Enable the optimization introduced in the previous patch for x86.
Best to make the patch description standalone, not referring to
"previous patch".
Good point. Will make the changelog standalone ;)
native_pv_tlb_init() checks whether native_flush_tlb_multi() is in use.
On CONFIG_PARAVIRT systems, it checks pv_ops; on non-PARAVIRT, native
flush is always in use.
It decides once at boot whether to enable the optimization: if using
native TLB flush and INVLPGB is not supported, we know IPIs were sent
and can skip the redundant sync. The decision is fixed via a static
key as Peter suggested[1].
PV backends (KVM, Xen, Hyper-V) typically have their own implementations
and don't call native_flush_tlb_multi() directly, so they cannot be trusted
to provide the IPI guarantees we need.
Two-step plan as David suggested[2]:
Step 1 (this patch): Skip redundant sync when we're 100% certain the TLB
flush sent IPIs. INVLPGB is excluded because when supported, we cannot
guarantee IPIs were sent, keeping it clean and simple.
Step 2 (future work): Send targeted IPIs only to CPUs actually doing
software/lockless page table walks, benefiting all architectures.
Regarding Step 2, it obviously only applies to setups where Step 1 does
not apply: like x86 with INVLPGB or arm64.
[1]
https://lore.kernel.org/linux-mm/[email protected]/
[2]
https://lore.kernel.org/linux-mm/[email protected]/
Suggested-by: Peter Zijlstra <[email protected]>
Suggested-by: David Hildenbrand (Arm) <[email protected]>
Signed-off-by: Lance Yang <[email protected]>
---
[...]
static inline void tlb_flush(struct mmu_gather *tlb)
{
unsigned long start = 0UL, end = TLB_FLUSH_ALL;
@@ -20,7 +30,12 @@ static inline void tlb_flush(struct mmu_gather *tlb)
end = tlb->end;
}
- flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
+ /*
+ * Pass both freed_tables and unshared_tables so that lazy-TLB CPUs
+ * also receive IPIs during unsharing page tables.
"unsharing of page tables" ?
Yes, that reads better.
I would maybe have it written ass
"Treat unshared_tables just like freed_tables, such that lazy-TLB CPUs
also receive IPIs during unsharing of page tables, allowing us to
safely implement tlb_table_flush_implies_ipi_broadcast()."
+ */
+ flush_tlb_mm_range(tlb->mm, start, end, stride_shift,
+ tlb->freed_tables || tlb->unshared_tables);
}
Cool, this wording is much clearer :)
In general, LGTM.
Acked-by: David Hildenbrand (Arm) <[email protected]>
Thanks for taking time to review!
