From: Sean Christopherson <sean.j.christopherson@intel•com>
To: lantianyu1986@gmail•com
Cc: , kvm@vger•kernel.org, rkrcmar@redhat•com,
catalin.marinas@arm•com, will.deacon@arm•com,
linux-kernel@vger•kernel.org, paulus@ozlabs•org, hpa@zytor•com,
kys@microsoft•com, kvmarm@lists•cs.columbia.edu,
mpe@ellerman•id.au, x86@kernel•org, linux@armlinux•org.uk,
michael.h.kelley@microsoft•com, mingo@redhat•com,
benh@kernel•crashing.org, jhogan@kernel•org,
linux-mips@vger•kernel.org, Lan Tianyu <Tianyu.Lan@microsoft•com>,
marc.zyngier@arm•com, kvm-ppc@vger•kernel.org, bp@alien8•de,
tglx@linutronix•de, linux-arm-kernel@lists•infradead.org,
christoffer.dall@arm•com, ralf@linux-mips•org,
paul.burton@mips•com, pbonzini@redhat•com, vkuznets@redhat•com,
linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH 7/11] KVM: Remove redundant check in the kvm_get_dirty_log_protect()
Date: Fri, 4 Jan 2019 07:50:36 -0800 [thread overview]
Message-ID: <20190104155036.GA11288@linux.intel.com> (raw)
In-Reply-To: <20190104085405.40356-8-Tianyu.Lan@microsoft.com>
On Fri, Jan 04, 2019 at 04:54:01PM +0800, lantianyu1986@gmail•com wrote:
> From: Lan Tianyu <Tianyu.Lan@microsoft•com>
>
> The dirty bits have already been checked in the previous check of
> "dirty_bitmap" and mask must be non-zero value at this point.
>
> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft•com>
> ---
> virt/kvm/kvm_main.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cf7cc0554094..e75dbb15fd09 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1206,11 +1206,9 @@ int kvm_get_dirty_log_protect(struct kvm *kvm,
> mask = xchg(&dirty_bitmap[i], 0);
> dirty_bitmap_buffer[i] = mask;
>
> - if (mask) {
> - offset = i * BITS_PER_LONG;
> - kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
> - offset, mask);
> - }
> + offset = i * BITS_PER_LONG;
> + kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
> + offset, mask);
Hmm, the check against mask was explicitly added by commit 58d2930f4ee3
("KVM: Eliminate extra function calls in kvm_get_dirty_log_protect()").
AFAIK KVM only *sets* bits in dirty_bitmap without holding slots_lock
and/or mmu_lock, so I agree that checking mask is redundant, but it'd be
nice to elaborate a bit more in the changelog.
At the very least this needs a Fixes tag for the aforementioned commit.
Tangentially related, does mmu_lock actually need to be held while we
walk dirty_bitmap in kvm_{clear,get}_dirty_log_protect()? The bitmap
itself is protected by slots_lock (a lockdep assertion would be nice
too), e.g. can we grab the lock iff dirty_bitmap[i] != 0?
> }
> spin_unlock(&kvm->mmu_lock);
> }
> --
> 2.14.4
>
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next prev parent reply other threads:[~2019-01-04 15:50 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-04 8:53 [PATCH 00/11] X86/KVM/Hyper-V: Add HV ept tlb range list flush support in KVM lantianyu1986
2019-01-04 8:53 ` [PATCH 1/11] X86/Hyper-V: Add parameter offset for hyperv_fill_flush_guest_mapping_list() lantianyu1986
2019-01-04 8:53 ` [PATCH 2/11] KVM/VMX: Fill range list in kvm_fill_hv_flush_list_func() lantianyu1986
2019-01-04 8:53 ` [PATCH 3/11] KVM: Add spte's point in the struct kvm_mmu_page lantianyu1986
2019-01-07 16:34 ` Paolo Bonzini
2019-01-04 8:53 ` [PATCH 4/11] KVM/MMU: Introduce tlb flush with range list lantianyu1986
2019-01-07 16:39 ` Paolo Bonzini
2019-01-04 8:53 ` [PATCH 5/11] KVM/MMU: Flush tlb directly in the kvm_mmu_slot_gfn_write_protect() lantianyu1986
2019-01-04 8:54 ` [PATCH 6/11] KVM/MMU: Flush tlb with range list in sync_page() lantianyu1986
2019-01-04 16:30 ` Sean Christopherson
2019-01-07 5:13 ` Tianyu Lan
2019-01-07 16:07 ` Paolo Bonzini
2019-01-04 8:54 ` [PATCH 7/11] KVM: Remove redundant check in the kvm_get_dirty_log_protect() lantianyu1986
2019-01-04 15:50 ` Sean Christopherson [this message]
2019-01-04 21:27 ` Sean Christopherson
2019-01-07 16:20 ` Paolo Bonzini
2019-01-04 8:54 ` [PATCH 8/11] KVM: Make kvm_arch_mmu_enable_log_dirty_pt_masked() return value lantianyu1986
2019-01-04 8:54 ` [PATCH 9/11] KVM/MMU: Flush tlb in the kvm_mmu_write_protect_pt_masked() lantianyu1986
2019-01-07 16:26 ` Paolo Bonzini
2019-01-10 9:06 ` Tianyu Lan
2019-01-04 8:54 ` [PATCH 10/11] KVM: Add flush parameter for kvm_age_hva() lantianyu1986
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