linux-kernel - Re: [PATCH v7 000/102] KVM TDX basic feature support

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Date:   Wed, 3 Aug 2022 18:48:45 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Nikunj A. Dadhania" <nikunj@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>, isaku.yamahata@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 000/102] KVM TDX basic feature support

On Wed, Jul 27, 2022 at 02:56:40PM +0530, Nikunj A. Dadhania wrote:
> On 7/26/2022 8:02 PM, Chao Peng wrote:
> > 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@...ux.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@...ux.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.
> 
> What if you do not have the tail lpage?
> 
> For example: memslot base_gfn = 0x1000 and npages is 0x800, so memslot range
> is 0x1000 to 0x17ff.
> 
> Assume a case when this function is called with start = 1000 and end = 1800.
> For 2M, page mask is 0x1ff. start and end both are 2M aligned.
> 
> First update_mixed takes care of 0x1000-0x1200
> Loop update_mixed: goes over from 0x1200 - 0x1800, there are no pages left
> for last update_mixed to process.

Oops, good catch. I would fix it differently by playing with lpage_end:
	lpage_end = (end - 1) & mask;

Thanks,
Chao

> 
> > 
> > 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));
> 
> lpage_info_slot some times causes a crash, as I noticed that
> lpage_info_slot() returns out-of-bound index.
> 
> Regards
> Nikunj
>