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KVM: x86: use again the flush argument of __link_shadow_page()

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Except in the case of parentless nested-TDP pages, mmu_page_zap_pte()
clears the SPTE but leaves the invalid_list empty.  In this case, using
kvm_flush_remote_tlbs() as kvm_mmu_remote_flush_or_zap() does is overkill.
Avoid flushing the entirety of the remote TLBs unless the invalid_list
was populated: instead, use a more efficient gfn-targeting flush (if
available) and skip it altogether if the caller guarantees that a TLB
flush is not necessary.

Based-on: <20260503201029.106481-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20260503210917.121840-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2026-05-03 23:09:17 +02:00
parent 87c810160e
commit 6b72d0578c
1 changed files with 21 additions and 2 deletions
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23
arch/x86/kvm/mmu/mmu.c
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@ -2526,6 +2526,23 @@ static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator)
__shadow_walk_next(iterator, *iterator->sptep);
}
/*
* Note: while normally KVM uses a "bool flush" return value to let
* the caller batch flushes, __link_shadow_page() flushes immediately
* before populating the parent PTE with the new shadow page. The
* typical callers, direct_map() and FNAME(fetch)(), are not going
* to zap more than one huge SPTE anyway.
*
* The only exception, where @flush can be false, is when a huge SPTE
* is replaced with a shadow page SPTE with a fully populated page table,
* which can happen from shadow_mmu_split_huge_page(). In this case,
* no memory is unmapped across the change to the page tables and no
* immediate flush is needed for correctness.
*
* Even in that case, calls to kvm_mmu_commit_zap_page() are not
* batched. Doing so would require adding an invalid_list argument
* all the way down to __walk_slot_rmaps().
*/
static void __link_shadow_page(struct kvm *kvm,
struct kvm_mmu_memory_cache *cache, u64 *sptep,
struct kvm_mmu_page *sp, bool flush)
@ -2541,8 +2558,10 @@ static void __link_shadow_page(struct kvm *kvm,
parent_sp = sptep_to_sp(sptep);
WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K);
mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list);
kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true);
if (mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list))
kvm_mmu_commit_zap_page(kvm, &invalid_list);
else if (flush)
kvm_flush_remote_tlbs_sptep(kvm, sptep);
}
spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));