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 16:36:16 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        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 07.07.22 16:26, Dave Hansen wrote:
> 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().

I think it is right.

Consider:

if (a)
     memblock_for_each_tdx_mem_pfn_range(...)
         func();
else
     other_func();


Juergen

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