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Date:   Tue, 26 Jul 2022 22:32:59 +0800
From:   Chao Peng <chao.p.peng@linux.intel.com>
To:     "Nikunj A. Dadhania" <nikunj@amd.com>
Cc:     Sean Christopherson <seanjc@google.com>, isaku.yamahata@intel.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        isaku.yamahata@gmail.com, Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v7 000/102] KVM TDX basic feature support
Message-ID: <20220726143259.GA323308@chaop.bj.intel.com>
Reply-To: Chao Peng <chao.p.peng@linux.intel.com>
References: <cover.1656366337.git.isaku.yamahata@intel.com>
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On Mon, Jul 25, 2022 at 07:16:24PM +0530, Nikunj A. Dadhania wrote:
> On 7/20/2022 8:29 PM, Chao Peng wrote:
> > On Thu, Jul 14, 2022 at 01:03:46AM +0000, Sean Christopherson wrote:
> > ...
> >>
> >> Option D). track shared regions in an Xarray, update kvm_arch_memory_slot.lpage_info
> >> on insertion/removal to (dis)allow hugepages as needed.
> >>
> >>   + efficient on KVM page fault (no new lookups)
> >>   + zero memory overhead (assuming KVM has to eat the cost of the Xarray anyways)
> >>   + straightforward to implement
> >>   + can (and should) be merged as part of the UPM series
> >>
> >> I believe xa_for_each_range() can be used to see if a given 2mb/1gb range is
> >> completely covered (fully shared) or not covered at all (fully private), but I'm
> >> not 100% certain that xa_for_each_range() works the way I think it does.
> > 
> > Hi Sean,
> > 
> > Below is the implementation to support 2M as you mentioned as option D.
> > It's based on UPM v7 xarray code: https://lkml.org/lkml/2022/7/6/259
> > 
> > Everything sounds good, the only trick bit is inc/dec disallow_lpage. If
> > we still treat it as a count, it will be a challenge to make the inc/dec
> > balanced. So in this patch I stole a bit for the purpose, looks ugly.
> > 
> > Any feedback is welcome.
> > 
> > Thanks,
> > Chao
> > 
> > -----------------------------------------------------------------------
> > From: Chao Peng <chao.p.peng@linux.intel.com>
> > Date: Wed, 20 Jul 2022 11:37:18 +0800
> > Subject: [PATCH] KVM: Add large page support for private memory
> > 
> > Update lpage_info when handling KVM_MEMORY_ENCRYPT_{UN,}REG_REGION.
> > 
> > Reserve a bit in disallow_lpage to indicate a large page has
> > private/share pages mixed.
> > 
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> 
> 
> > +static void update_mem_lpage_info(struct kvm *kvm,
> > +				  struct kvm_memory_slot *slot,
> > +				  unsigned int attr,
> > +				  gfn_t start, gfn_t end)
> > +{
> > +	unsigned long lpage_start, lpage_end;
> > +	unsigned long gfn, pages, mask;
> > +	int level;
> > +
> > +	for (level = PG_LEVEL_2M; level <= KVM_MAX_HUGEPAGE_LEVEL; level++) {
> > +		pages = KVM_PAGES_PER_HPAGE(level);
> > +		mask = ~(pages - 1);
> > +		lpage_start = start & mask;
> > +		lpage_end = end & mask;
> > +
> > +		/*
> > +		 * We only need to scan the head and tail page, for middle pages
> > +		 * we know they are not mixed.
> > +		 */
> > +		update_mixed(lpage_info_slot(lpage_start, slot, level),
> > +			     mem_attr_is_mixed(kvm, attr, lpage_start,
> > +							  lpage_start + pages));
> > +
> > +		if (lpage_start == lpage_end)
> > +			return;
> > +
> > +		for (gfn = lpage_start + pages; gfn < lpage_end; gfn += pages) {
> > +			update_mixed(lpage_info_slot(gfn, slot, level), false);
> > +		}
> 
> Boundary check missing here for the case when gfn reaches lpage_end.
> 
> 		if (gfn == lpage_end)
> 			return;

In this case, it's actually the tail page that I want to scan for with
below code.

It's also possible I misunderstand something here.

Chao
> 
> > +
> > +		update_mixed(lpage_info_slot(lpage_end, slot, level),
> > +			     mem_attr_is_mixed(kvm, attr, lpage_end,
> > +							  lpage_end + pages));
> > +	}
> > +}
> 
> Regards
> Nikunj