Re: [PATCH 1/7] KVM: arm64: Add two page mapping counters for kvm_stat

From: Marc Zyngier <maz@kernel.org>
To: Keqian Zhu <zhukeqian1@huawei.com>,  Yoan Picchi <yoan.picchi@arm.com>
Cc: <james.morse@arm.com>, <julien.thierry.kdev@gmail.com>,
	<suzuki.poulose@arm.com>, <linux-arm-kernel@lists.infradead.org>,
	<kvmarm@lists.cs.columbia.edu>, <catalin.marinas@arm.com>,
	<will@kernel.org>
Subject: Re: [PATCH 1/7] KVM: arm64: Add two page mapping counters for kvm_stat
Date: Tue, 23 Mar 2021 17:03:02 +0000	[thread overview]
Message-ID: <87tup1kdkp.wl-maz@kernel.org> (raw)
In-Reply-To: <fe120fea-b2a3-d80f-38df-2430a0b2029e@huawei.com>

On Tue, 23 Mar 2021 11:52:38 +0000,
Keqian Zhu <zhukeqian1@huawei.com> wrote:
> 
> Hi Yoan,
> 
> On 2021/3/20 0:17, Yoan Picchi wrote:
> > Add a counter for when a regular page is mapped, and another
> > for when a huge page is mapped.
> > 
> > Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h | 2 ++
> >  arch/arm64/kvm/guest.c            | 2 ++
> >  arch/arm64/kvm/mmu.c              | 7 +++++++
> >  3 files changed, 11 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 3d10e6527..863603285 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -561,6 +561,8 @@ struct kvm_vcpu_stat {
> >  	u64 wfi_exit_stat;
> >  	u64 mmio_exit_user;
> >  	u64 mmio_exit_kernel;
> > +	u64 regular_page_mapped;
> > +	u64 huge_page_mapped;
> >  	u64 exits;
> >  };
> >  
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index 9bbd30e62..14b15fb8f 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -38,6 +38,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
> >  	VCPU_STAT("wfi_exit_stat", wfi_exit_stat),
> >  	VCPU_STAT("mmio_exit_user", mmio_exit_user),
> >  	VCPU_STAT("mmio_exit_kernel", mmio_exit_kernel),
> > +	VCPU_STAT("regular_page_mapped", regular_page_mapped),
> > +	VCPU_STAT("huge_page_mapped", huge_page_mapped),
> >  	VCPU_STAT("exits", exits),
> >  	VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
> >  	VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 77cb2d28f..3996b28da 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -914,6 +914,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >  		mark_page_dirty(kvm, gfn);
> >  	}
> >  
> > +	if (ret >= 0) {
> > +		if (vma_pagesize == PAGE_SIZE) {
> > +			vcpu->stat.regular_page_mapped++;
> > +		} else {
> > +			vcpu->stat.huge_page_mapped++;
> > +		}
> > +	}
> This looks too simple, as we don't always map a new regular_page or
> huge_page here.  We may just relax permission or change mapping
> size, etc. If we mix these operations, the stat result seems not
> very valuable...

I can only agree. There is no such thing as a *huge page*. We have a
whole gamut of block mappings depending on the base granule size, and
we even have concatenation of pages/blocks (not yet implemented at S2,
but there is hope...).

What is this trying to express?

Thanks,

	M.

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

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