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, 07 Jul 2022 14:37:46 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...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


> > > +/*
> > > + * 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().
> 
> 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.

Hi Dave,

Sorry to ping. Just double check, any comments around here, ..

> 
> > Or, should we be doing something different with the 1MB in the memblock
> > structure?
> 
> memblock APIs are used by other kernel components.  I don't think we should
> modify memblock code behaviour for TDX.  Do you have any specific suggestion?
> 
> One possible option I can think is explicitly "register" memory regions as TDX
> memory when they are firstly freed to the page allocator.  

[...]

> 
> This will require new data structures to represent TDX memblock and the code to
> create, insert and merge contiguous TDX memblocks, etc.  The advantage is we can
> just iterate those TDX memblocks when constructing TDMRs.
> 
> 

And here?