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

[Juergen Gross <jgross@xxxxxxxx>]

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: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature