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

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Date:   Mon, 25 Jul 2022 19:16:24 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 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;

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

Regards
Nikunj