Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

From: Chao Peng <chao.p.peng@linux.intel.com>
To: Vishal Annapurve <vannapurve@google.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>,
	Sean Christopherson <seanjc@google.com>,
	Michael Roth <michael.roth@amd.com>,
	"Nikunj A. Dadhania" <nikunj@amd.com>,
	kvm list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-api@vger.kernel.org, linux-doc@vger.kernel.org,
	qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
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	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
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	David Hildenbrand <david@redhat.com>,
	aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com,
	Quentin Perret <qperret@google.com>,
	mhocko@suse.com
Subject: Re: [PATCH v6 6/8] KVM: Handle page fault for private memory
Date: Thu, 21 Jul 2022 17:45:23 +0800	[thread overview]
Message-ID: <20220721094523.GC153288@chaop.bj.intel.com> (raw)
In-Reply-To: <CAGtprH9knCr++C7jgXYCi1zfYcreip1uun-d+eucjEQy9xymNg@mail.gmail.com>

On Wed, Jul 20, 2022 at 04:08:10PM -0700, Vishal Annapurve wrote:
> > > Hmm, so a new slot->arch.page_attr array shouldn't be necessary, KVM can instead
> > > update slot->arch.lpage_info on shared<->private conversions.  Detecting whether
> > > a given range is partially mapped could get nasty if KVM defers tracking to the
> > > backing store, but if KVM itself does the tracking as was previously suggested[*],
> > > then updating lpage_info should be relatively straightfoward, e.g. use
> > > xa_for_each_range() to see if a given 2mb/1gb range is completely covered (fully
> > > shared) or not covered at all (fully private).
> > >
> > > [*] https://lore.kernel.org/all/YofeZps9YXgtP3f1@google.com
> >
> > Yes, slot->arch.page_attr was introduced to help identify whether a page
> > is completely shared/private at given level. It seems XARRAY can serve
> > the same purpose, though I know nothing about it. Looking forward to
> > seeing the patch of using XARRAY.
> >
> > yes, update slot->arch.lpage_info is good to utilize the existing logic
> > and Isaku has applied it to slot->arch.lpage_info for 2MB support patches.
> 
> Chao, are you planning to implement these changes to ensure proper
> handling of hugepages partially mapped as private/shared in subsequent
> versions of this series?
> Or is this something left to be handled by the architecture specific code?

Ah, the topic gets moved to a different place. I should update here.
There were more discussions under TDX KVM patch series and I actually
just sent out the draft code for this:

https://lkml.org/lkml/2022/7/20/610

That patch is based on UPM v7 here. If I can get more feedbacks there
then I will include an udpated version in UPM v8.

If you have bandwdith, you can also play with that patch, any feedback
is welcome.

Chao
> 
> Regards,
> Vishal