linux-kernel - Re: [PATCH 2/2] KVM: arm64: Skip the cache flush when coalescing tables into a block

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Message-ID: <87y2ewyawn.wl-maz@kernel.org>
Date:   Tue, 09 Mar 2021 08:43:52 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     "wangyanan (Y)" <wangyanan55@...wei.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <wanghaibin.wang@...wei.com>,
        <yuzenghui@...wei.com>
Subject: Re: [PATCH 2/2] KVM: arm64: Skip the cache flush when coalescing tables into a block

On Tue, 09 Mar 2021 08:34:43 +0000,
"wangyanan (Y)" <wangyanan55@...wei.com> wrote:
> 
> 
> On 2021/3/9 0:34, Will Deacon wrote:
> > On Mon, Jan 25, 2021 at 10:10:44PM +0800, Yanan Wang wrote:
> >> After dirty-logging is stopped for a VM configured with huge mappings,
> >> KVM will recover the table mappings back to block mappings. As we only
> >> replace the existing page tables with a block entry and the cacheability
> >> has not been changed, the cache maintenance opreations can be skipped.
> >> 
> >> Signed-off-by: Yanan Wang <wangyanan55@...wei.com>
> >> ---
> >>   arch/arm64/kvm/mmu.c | 12 +++++++++---
> >>   1 file changed, 9 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >> index 8e8549ea1d70..37b427dcbc4f 100644
> >> --- a/arch/arm64/kvm/mmu.c
> >> +++ b/arch/arm64/kvm/mmu.c
> >> @@ -744,7 +744,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >>   {
> >>   	int ret = 0;
> >>   	bool write_fault, writable, force_pte = false;
> >> -	bool exec_fault;
> >> +	bool exec_fault, adjust_hugepage;
> >>   	bool device = false;
> >>   	unsigned long mmu_seq;
> >>   	struct kvm *kvm = vcpu->kvm;
> >> @@ -872,12 +872,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >>   		mark_page_dirty(kvm, gfn);
> >>   	}
> >>   -	if (fault_status != FSC_PERM && !device)
> >> +	/*
> >> +	 * There is no necessity to perform cache maintenance operations if we
> >> +	 * will only replace the existing table mappings with a block mapping.
> >> +	 */
> >> +	adjust_hugepage = fault_granule < vma_pagesize ? true : false;
> > nit: you don't need the '? true : false' part
> > 
> > That said, your previous patch checks for 'fault_granule > vma_pagesize',
> > so I'm not sure the local variable helps all that much here because it
> > obscures the size checks in my opinion. It would be more straight-forward
> > if we could structure the logic as:
> > 
> > 
> > 	if (fault_granule < vma_pagesize) {
> > 
> > 	} else if (fault_granule > vma_page_size) {
> > 
> > 	} else {
> > 
> > 	}
> > 
> > With some comments describing what we can infer about the memcache and cache
> > maintenance requirements for each case.
> Thanks for your suggestion here, Will.
> But I have resent another newer series [1] (KVM: arm64: Improve
> efficiency of stage2 page table)
> recently, which has the same theme but different solutions that I
> think are better.
> [1]
> https://lore.kernel.org/lkml/20210208112250.163568-1-wangyanan55@huawei.com/
> 
> Could you please comment on that series ?  I think it can be found in
> your inbox :).

There were already a bunch of comments on that series, and I stopped
at the point where the cache maintenance was broken. Please respin
that series if you want further feedback on it.

In the future, if you deprecate a series (which is completely
understandable), please leave a note on the list with a pointer to the
new series so that people don't waste time reviewing an obsolete
series. Or post the new series with a new version number so that it is
obvious that the original series has been superseded.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.