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Message-ID: <ed34e089-5a35-2502-5a7d-ad8b1cf6957f@oracle.com>
Date: Fri, 22 Oct 2021 13:44:44 +0200
From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
To: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 2/3] KVM: x86/mmu: Drop a redundant remote TLB flush in
kvm_zap_gfn_range()
On 22.10.2021 03:00, Sean Christopherson wrote:
> Remove an unnecessary remote TLB flush in kvm_zap_gfn_range() now that
> said function holds mmu_lock for write for its entire duration. The
> flush was added by the now-reverted commit to allow TDP MMU to flush while
> holding mmu_lock for read, as the transition from write=>read required
> dropping the lock and thus a pending flush needed to be serviced.
>
> Fixes: 5a324c24b638 ("Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"")
> Cc: Maxim Levitsky <mlevitsk@...hat.com>
> Cc: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> Cc: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 3 ---
> 1 file changed, 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index f82b192bba0b..e8b8a665e2e9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5700,9 +5700,6 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> end - 1, true, flush);
> }
> }
> - if (flush)
> - kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> - gfn_end - gfn_start);
> }
>
> if (is_tdp_mmu_enabled(kvm)) {
>
Unfortunately, it seems that a pending flush from __kvm_zap_rmaps()
can be reset back to false by the following line:
> flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start, gfn_end, flush);
kvm_tdp_mmu_zap_gfn_range() calls __kvm_tdp_mmu_zap_gfn_range with
"can_yield" set to true, which passes it to zap_gfn_range, which has
this code:
> if (can_yield &&
> tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
> flush = false;
> continue;
> }
Thanks,
Maciej
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