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

[Juergen Gross <jgross@suse.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 2630 bytes --]

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

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3149 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]