linux-kernel - Re: [PATCH v5 12/22] x86/virt/tdx: Convert all memory regions in memblock to TDX memory

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Date:   Thu, 7 Jul 2022 07:26:30 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v5 12/22] x86/virt/tdx: Convert all memory regions in
 memblock to TDX memory

On 6/26/22 23:16, Kai Huang wrote:
> On Fri, 2022-06-24 at 12:40 -0700, Dave Hansen wrote:
>>> +/*
>>> + * Walks over all memblock memory regions that are intended to be
>>> + * converted to TDX memory.  Essentially, it is all memblock memory
>>> + * regions excluding the low memory below 1MB.
>>> + *
>>> + * This is because on some TDX platforms the low memory below 1MB is
>>> + * not included in CMRs.  Excluding the low 1MB can still guarantee
>>> + * that the pages managed by the page allocator are always TDX memory,
>>> + * as the low 1MB is reserved during kernel boot and won't end up to
>>> + * the ZONE_DMA (see reserve_real_mode()).
>>> + */
>>> +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid)	\
>>> +	for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid)	\
>>> +		if (!pfn_range_skip_lowmem(p_start, p_end))
>>
>> Let's summarize where we are at this point:
>>
>> 1. All RAM is described in memblocks
>> 2. Some memblocks are reserved and some are free
>> 3. The lower 1MB is marked reserved
>> 4. for_each_mem_pfn_range() walks all reserved and free memblocks, so we
>>    have to exclude the lower 1MB as a special case.
>>
>> That seems superficially rather ridiculous.  Shouldn't we just pick a
>> memblock iterator that skips the 1MB?  Surely there is such a thing.
> 
> Perhaps you are suggesting we should always loop the _free_ ranges so we don't
> need to care about the first 1MB which is reserved?
> 
> The problem is some reserved memory regions are actually later freed to the page
> allocator, for example, initrd.  So to cover all those 'late-freed-reserved-
> regions', I used for_each_mem_pfn_range(), instead of for_each_free_mem_range().

Why not just entirely remove the lower 1MB from the memblock structure
on TDX systems?  Do something equivalent to adding this on the kernel
command line:

	memmap=1M$0x0

> Btw, I do have a checkpatch warning around this code:
> 
> ERROR: Macros with complex values should be enclosed in parentheses
> #109: FILE: arch/x86/virt/vmx/tdx/tdx.c:377:
> +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid)	\
> +	for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid)	\
> +		if (!pfn_range_skip_lowmem(p_start, p_end))
> 
> But it looks like a false positive to me.

I think it doesn't like the if().